diff --git a/dom/base/nsFrameMessageManager.cpp b/dom/base/nsFrameMessageManager.cpp
index a14b0e058336..d7ff43d8c086 100644
--- a/dom/base/nsFrameMessageManager.cpp
+++ b/dom/base/nsFrameMessageManager.cpp
@@ -21,6 +21,7 @@
 #include "js/CallAndConstruct.h"  // JS::IsCallable, JS_CallFunctionValue
 #include "js/CompilationAndEvaluation.h"
 #include "js/CompileOptions.h"
+#include "js/EnvironmentChain.h"  // JS::EnvironmentChain
 #include "js/experimental/JSStencil.h"
 #include "js/GCVector.h"
 #include "js/JSON.h"
@@ -1212,7 +1213,7 @@ void nsMessageManagerScriptExecutor::LoadScriptInternal(
         }
       } else {
         JS::Rooted<JS::Value> rval(cx);
-        JS::RootedVector<JSObject*> envChain(cx);
+        JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
         if (!envChain.append(aMessageManager)) {
           return;
         }
diff --git a/dom/base/nsJSUtils.cpp b/dom/base/nsJSUtils.cpp
index 48df3ae2d30b..bf7eb34da03c 100644
--- a/dom/base/nsJSUtils.cpp
+++ b/dom/base/nsJSUtils.cpp
@@ -19,6 +19,7 @@
 #include "js/CompilationAndEvaluation.h"
 #include "js/CompileOptions.h"
 #include "js/Date.h"
+#include "js/EnvironmentChain.h"
 #include "js/GCVector.h"
 #include "js/HeapAPI.h"
 #include "js/Modules.h"
@@ -80,7 +81,7 @@ nsresult nsJSUtils::UpdateFunctionDebugMetadata(
 }
 
 nsresult nsJSUtils::CompileFunction(AutoJSAPI& jsapi,
-                                    JS::HandleVector<JSObject*> aScopeChain,
+                                    const JS::EnvironmentChain& aEnvChain,
                                     JS::CompileOptions& aOptions,
                                     const nsACString& aName, uint32_t aArgCount,
                                     const char** aArgArray,
@@ -88,12 +89,12 @@ nsresult nsJSUtils::CompileFunction(AutoJSAPI& jsapi,
                                     JSObject** aFunctionObject) {
   JSContext* cx = jsapi.cx();
   MOZ_ASSERT(js::GetContextRealm(cx));
-  MOZ_ASSERT_IF(aScopeChain.length() != 0,
-                js::IsObjectInContextCompartment(aScopeChain[0], cx));
+  MOZ_ASSERT_IF(aEnvChain.length() != 0,
+                js::IsObjectInContextCompartment(aEnvChain.chain()[0], cx));
 
   // Do the junk Gecko is supposed to do before calling into JSAPI.
-  for (size_t i = 0; i < aScopeChain.length(); ++i) {
-    JS::ExposeObjectToActiveJS(aScopeChain[i]);
+  for (size_t i = 0; i < aEnvChain.length(); ++i) {
+    JS::ExposeObjectToActiveJS(aEnvChain.chain()[i]);
   }
 
   // Compile.
@@ -106,7 +107,7 @@ nsresult nsJSUtils::CompileFunction(AutoJSAPI& jsapi,
   }
 
   JS::Rooted<JSFunction*> fun(
-      cx, JS::CompileFunction(cx, aScopeChain, aOptions,
+      cx, JS::CompileFunction(cx, aEnvChain, aOptions,
                               PromiseFlatCString(aName).get(), aArgCount,
                               aArgArray, source));
   if (!fun) {
@@ -122,14 +123,14 @@ bool nsJSUtils::IsScriptable(JS::Handle<JSObject*> aEvaluationGlobal) {
   return xpc::Scriptability::AllowedIfExists(aEvaluationGlobal);
 }
 
-static bool AddScopeChainItem(JSContext* aCx, nsINode* aNode,
-                              JS::MutableHandleVector<JSObject*> aScopeChain) {
+static bool AddEnvChainItem(JSContext* aCx, nsINode* aNode,
+                            JS::EnvironmentChain& aEnvChain) {
   JS::Rooted<JS::Value> val(aCx);
   if (!GetOrCreateDOMReflector(aCx, aNode, &val)) {
     return false;
   }
 
-  if (!aScopeChain.append(&val.toObject())) {
+  if (!aEnvChain.append(&val.toObject())) {
     return false;
   }
 
@@ -137,11 +138,10 @@ static bool AddScopeChainItem(JSContext* aCx, nsINode* aNode,
 }
 
 /* static */
-bool nsJSUtils::GetScopeChainForElement(
-    JSContext* aCx, Element* aElement,
-    JS::MutableHandleVector<JSObject*> aScopeChain) {
+bool nsJSUtils::GetEnvironmentChainForElement(JSContext* aCx, Element* aElement,
+                                              JS::EnvironmentChain& aEnvChain) {
   for (nsINode* cur = aElement; cur; cur = cur->GetScopeChainParent()) {
-    if (!AddScopeChainItem(aCx, cur, aScopeChain)) {
+    if (!AddEnvChainItem(aCx, cur, aEnvChain)) {
       return false;
     }
   }
diff --git a/dom/base/nsJSUtils.h b/dom/base/nsJSUtils.h
index 8b4c1492c648..f32e21752d50 100644
--- a/dom/base/nsJSUtils.h
+++ b/dom/base/nsJSUtils.h
@@ -29,6 +29,10 @@ class nsIScriptElement;
 class nsIScriptGlobalObject;
 class nsXBLPrototypeBinding;
 
+namespace JS {
+class JS_PUBLIC_API EnvironmentChain;
+};
+
 namespace mozilla {
 union Utf8Unit;
 
@@ -51,7 +55,7 @@ class nsJSUtils {
   static uint64_t GetCurrentlyRunningCodeInnerWindowID(JSContext* aContext);
 
   static nsresult CompileFunction(mozilla::dom::AutoJSAPI& jsapi,
-                                  JS::HandleVector<JSObject*> aScopeChain,
+                                  const JS::EnvironmentChain& aEnvChain,
                                   JS::CompileOptions& aOptions,
                                   const nsACString& aName, uint32_t aArgCount,
                                   const char** aArgArray,
@@ -66,10 +70,10 @@ class nsJSUtils {
   static bool IsScriptable(JS::Handle<JSObject*> aEvaluationGlobal);
 
   // Returns false if an exception got thrown on aCx.  Passing a null
-  // aElement is allowed; that wil produce an empty aScopeChain.
-  static bool GetScopeChainForElement(
-      JSContext* aCx, mozilla::dom::Element* aElement,
-      JS::MutableHandleVector<JSObject*> aScopeChain);
+  // aElement is allowed; that wil produce an empty aEnvChain.
+  static bool GetEnvironmentChainForElement(JSContext* aCx,
+                                            mozilla::dom::Element* aElement,
+                                            JS::EnvironmentChain& aEnvChain);
 
   static void ResetTimeZone();
 
diff --git a/dom/events/EventListenerManager.cpp b/dom/events/EventListenerManager.cpp
index 19d7e58af177..57ff22b76e5c 100644
--- a/dom/events/EventListenerManager.cpp
+++ b/dom/events/EventListenerManager.cpp
@@ -7,7 +7,8 @@
 // Microsoft's API Name hackery sucks
 #undef CreateEvent
 
-#include "js/ColumnNumber.h"  // JS::ColumnNumberOneOrigin
+#include "js/ColumnNumber.h"      // JS::ColumnNumberOneOrigin
+#include "js/EnvironmentChain.h"  // JS::EnvironmentChain
 #include "js/loader/LoadedScript.h"
 #include "js/loader/ScriptFetchOptions.h"
 #include "mozilla/Assertions.h"
@@ -1219,12 +1220,12 @@ nsresult EventListenerManager::CompileEventHandlerInternal(
   JSAutoRealm ar(cx, target);
 
   // Now that we've entered the realm we actually care about, create our
-  // scope chain.  Note that we start with |element|, not aElement, because
-  // mTarget is different from aElement in the <body> case, where mTarget is a
-  // Window, and in that case we do not want the scope chain to include the body
-  // or the document.
-  JS::RootedVector<JSObject*> scopeChain(cx);
-  if (!nsJSUtils::GetScopeChainForElement(cx, element, &scopeChain)) {
+  // environment chain.  Note that we start with |element|, not aElement,
+  // because mTarget is different from aElement in the <body> case, where
+  // mTarget is a Window, and in that case we do not want the environment chain
+  // to include the body or the document.
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::Yes);
+  if (!nsJSUtils::GetEnvironmentChainForElement(cx, element, envChain)) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
@@ -1257,7 +1258,7 @@ nsresult EventListenerManager::CompileEventHandlerInternal(
       .setDeferDebugMetadata(true);
 
   JS::Rooted<JSObject*> handler(cx);
-  result = nsJSUtils::CompileFunction(jsapi, scopeChain, options,
+  result = nsJSUtils::CompileFunction(jsapi, envChain, options,
                                       nsAtomCString(aTypeAtom), argCount,
                                       argNames, *body, handler.address());
   NS_ENSURE_SUCCESS(result, result);
diff --git a/js/public/CompilationAndEvaluation.h b/js/public/CompilationAndEvaluation.h
index fee0aecbb09d..c8e64b007d22 100644
--- a/js/public/CompilationAndEvaluation.h
+++ b/js/public/CompilationAndEvaluation.h
@@ -27,6 +27,7 @@ union Utf8Unit;
 
 namespace JS {
 
+class JS_PUBLIC_API EnvironmentChain;
 class JS_PUBLIC_API InstantiateOptions;
 class JS_PUBLIC_API ReadOnlyCompileOptions;
 
@@ -84,12 +85,12 @@ extern JS_PUBLIC_API bool JS_ExecuteScript(JSContext* cx,
  * objects that should end up on the script's scope chain.
  */
 extern JS_PUBLIC_API bool JS_ExecuteScript(JSContext* cx,
-                                           JS::HandleObjectVector envChain,
+                                           const JS::EnvironmentChain& envChain,
                                            JS::Handle<JSScript*> script,
                                            JS::MutableHandle<JS::Value> rval);
 
 extern JS_PUBLIC_API bool JS_ExecuteScript(JSContext* cx,
-                                           JS::HandleObjectVector envChain,
+                                           const JS::EnvironmentChain& envChain,
                                            JS::Handle<JSScript*> script);
 
 namespace JS {
@@ -108,7 +109,8 @@ extern JS_PUBLIC_API bool Evaluate(JSContext* cx,
  * the global object on it; that's implicit.  It needs to contain the other
  * objects that should end up on the script's scope chain.
  */
-extern JS_PUBLIC_API bool Evaluate(JSContext* cx, HandleObjectVector envChain,
+extern JS_PUBLIC_API bool Evaluate(JSContext* cx,
+                                   const JS::EnvironmentChain& envChain,
                                    const ReadOnlyCompileOptions& options,
                                    SourceText<char16_t>& srcBuf,
                                    MutableHandle<Value> rval);
@@ -173,7 +175,7 @@ extern JS_PUBLIC_API JSScript* CompileUtf8Path(
  * global must not be explicitly included in the scope chain.
  */
 extern JS_PUBLIC_API JSFunction* CompileFunction(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const JS::EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, SourceText<char16_t>& srcBuf);
 
@@ -185,7 +187,7 @@ extern JS_PUBLIC_API JSFunction* CompileFunction(
  * global must not be explicitly included in the scope chain.
  */
 extern JS_PUBLIC_API JSFunction* CompileFunction(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const JS::EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, SourceText<mozilla::Utf8Unit>& srcBuf);
 
@@ -194,7 +196,7 @@ extern JS_PUBLIC_API JSFunction* CompileFunction(
  * Rust-friendly ergonomics.
  */
 extern JS_PUBLIC_API JSFunction* CompileFunctionUtf8(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const JS::EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, const char* utf8, size_t length);
 
diff --git a/js/public/EnvironmentChain.h b/js/public/EnvironmentChain.h
new file mode 100644
index 000000000000..7d37184b521b
--- /dev/null
+++ b/js/public/EnvironmentChain.h
@@ -0,0 +1,67 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef js_EnvironmentChain_h
+#define js_EnvironmentChain_h
+
+#include "mozilla/Attributes.h"  // MOZ_RAII
+
+#include <stddef.h>  // size_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+#include "js/GCVector.h"  // JS::RootedVector
+
+struct JS_PUBLIC_API JSContext;
+class JS_PUBLIC_API JSObject;
+
+namespace JS {
+
+enum class SupportUnscopables : bool { No = false, Yes = true };
+
+/**
+ * JS::EnvironmentChain stores a list of objects to put on the environment
+ * chain.
+ *
+ * Internally the engine will create a non-syntactic 'with' environment for each
+ * of these objects. Note that 'with' environments aren't optimized well so you
+ * should use this class only if you really have to.
+ *
+ * The SupportUnscopables enum class controls whether these non-syntactic 'with'
+ * environments support Symbol.unscopables similar to syntactic 'with'
+ * statements in JS.
+ *
+ * Passing SupportUnscopables::No is better for performance because it lets us
+ * skip the Symbol.unscopables property lookup. Some Web APIs require supporting
+ * Symbol.unscopables though. In Firefox, SupportUnscopables::Yes is used for
+ * event handlers.
+ */
+class MOZ_RAII JS_PUBLIC_API EnvironmentChain {
+  JS::RootedObjectVector chain_;
+  SupportUnscopables supportUnscopables_;
+
+ public:
+  EnvironmentChain(JSContext* cx, SupportUnscopables supportUnscopables)
+      : chain_(cx), supportUnscopables_(supportUnscopables) {}
+
+  EnvironmentChain(const EnvironmentChain&) = delete;
+  void operator=(const EnvironmentChain&) = delete;
+
+  [[nodiscard]] bool append(JSObject* obj) { return chain_.append(obj); }
+  bool empty() const { return chain_.empty(); }
+  size_t length() const { return chain_.length(); }
+
+  RootedObjectVector& chain() { return chain_; }
+  const RootedObjectVector& chain() const { return chain_; }
+
+  void setSupportUnscopables(SupportUnscopables supportUnscopables) {
+    supportUnscopables_ = supportUnscopables;
+  }
+  SupportUnscopables supportUnscopables() const { return supportUnscopables_; }
+};
+
+}  // namespace JS
+
+#endif /* js_EnvironmentChain_h */
diff --git a/js/public/friend/JSMEnvironment.h b/js/public/friend/JSMEnvironment.h
index ad16f2ba6260..36d3337c971b 100644
--- a/js/public/friend/JSMEnvironment.h
+++ b/js/public/friend/JSMEnvironment.h
@@ -45,6 +45,8 @@
 
 namespace JS {
 
+class JS_PUBLIC_API EnvironmentChain;
+
 /**
  * Allocate a new environment in the current compartment that is compatible with
  * JSM shared loading.
@@ -68,7 +70,7 @@ extern JS_PUBLIC_API bool ExecuteInJSMEnvironment(JSContext* cx,
 // temporarily placed on the environment chain.
 extern JS_PUBLIC_API bool ExecuteInJSMEnvironment(
     JSContext* cx, Handle<JSScript*> script, Handle<JSObject*> jsmEnv,
-    Handle<StackGCVector<JSObject*>> targetObj);
+    const EnvironmentChain& targetObj);
 
 // Used by native methods to determine the JSMEnvironment of caller if possible
 // by looking at stack frames. Returns nullptr if top frame isn't a scripted
diff --git a/js/src/builtin/Eval.cpp b/js/src/builtin/Eval.cpp
index 2100bf3648a3..04ebbd023d54 100644
--- a/js/src/builtin/Eval.cpp
+++ b/js/src/builtin/Eval.cpp
@@ -12,6 +12,7 @@
 #include "frontend/BytecodeCompiler.h"  // frontend::CompileEvalScript
 #include "gc/HashUtil.h"
 #include "js/CompilationAndEvaluation.h"
+#include "js/EnvironmentChain.h"       // JS::EnvironmentChain
 #include "js/friend/ErrorMessages.h"   // js::GetErrorMessage, JSMSG_*
 #include "js/friend/JSMEnvironment.h"  // JS::NewJSMEnvironment, JS::ExecuteInJSMEnvironment, JS::GetJSMEnvironmentOfScriptedCaller, JS::IsJSMEnvironment
 #include "js/friend/WindowProxy.h"     // js::IsWindowProxy
@@ -410,7 +411,7 @@ JS_PUBLIC_API bool js::ExecuteInFrameScriptEnvironment(
     return false;
   }
 
-  RootedObjectVector envChain(cx);
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
   if (!envChain.append(objArg)) {
     return false;
   }
@@ -461,14 +462,13 @@ JS_PUBLIC_API JSObject* JS::NewJSMEnvironment(JSContext* cx) {
 JS_PUBLIC_API bool JS::ExecuteInJSMEnvironment(JSContext* cx,
                                                HandleScript scriptArg,
                                                HandleObject varEnv) {
-  RootedObjectVector emptyChain(cx);
+  JS::EnvironmentChain emptyChain(cx, JS::SupportUnscopables::No);
   return ExecuteInJSMEnvironment(cx, scriptArg, varEnv, emptyChain);
 }
 
-JS_PUBLIC_API bool JS::ExecuteInJSMEnvironment(JSContext* cx,
-                                               HandleScript scriptArg,
-                                               HandleObject varEnv,
-                                               HandleObjectVector targetObj) {
+JS_PUBLIC_API bool JS::ExecuteInJSMEnvironment(
+    JSContext* cx, HandleScript scriptArg, HandleObject varEnv,
+    const EnvironmentChain& targetObj) {
   cx->check(varEnv);
   MOZ_ASSERT(
       ObjectRealm::get(varEnv).getNonSyntacticLexicalEnvironment(varEnv));
diff --git a/js/src/debugger/Frame.cpp b/js/src/debugger/Frame.cpp
index e4f555750e77..c5c15e2be8d2 100644
--- a/js/src/debugger/Frame.cpp
+++ b/js/src/debugger/Frame.cpp
@@ -41,6 +41,7 @@
 #include "jit/JSJitFrameIter.h"         // for InlineFrameIterator
 #include "jit/RematerializedFrame.h"    // for RematerializedFrame
 #include "js/CallArgs.h"                // for CallArgs
+#include "js/EnvironmentChain.h"        // JS::EnvironmentChain
 #include "js/friend/ErrorMessages.h"    // for GetErrorMessage, JSMSG_*
 #include "js/GCVector.h"                // for JS::StackGCVector
 #include "js/Object.h"                  // for SetReservedSlot
@@ -954,7 +955,7 @@ static WithEnvironmentObject* CreateBindingsEnv(
     }
   }
 
-  RootedObjectVector envChain(cx);
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
   if (!envChain.append(bindingsObj)) {
     return nullptr;
   }
diff --git a/js/src/frontend/BytecodeCompiler.cpp b/js/src/frontend/BytecodeCompiler.cpp
index 453a3107cc5f..ffebc0eba928 100644
--- a/js/src/frontend/BytecodeCompiler.cpp
+++ b/js/src/frontend/BytecodeCompiler.cpp
@@ -28,6 +28,7 @@
 #include "js/AllocPolicy.h"        // js::SystemAllocPolicy, ReportOutOfMemory
 #include "js/CharacterEncoding.h"  // JS_EncodeStringToUTF8
 #include "js/ColumnNumber.h"       // JS::ColumnNumberOneOrigin
+#include "js/EnvironmentChain.h"   // JS::SupportUnscopables
 #include "js/ErrorReport.h"        // JS_ReportErrorASCII
 #include "js/experimental/JSStencil.h"
 #include "js/GCVector.h"    // JS::StackGCVector
@@ -612,8 +613,8 @@ static WithEnvironmentObject* CreateExtraBindingsEnvironment(
   }
 
   JS::Rooted<JSObject*> globalLexical(cx, &cx->global()->lexicalEnvironment());
-  return WithEnvironmentObject::createNonSyntactic(cx, extraBindingsObj,
-                                                   globalLexical);
+  return WithEnvironmentObject::createNonSyntactic(
+      cx, extraBindingsObj, globalLexical, JS::SupportUnscopables::No);
 }
 
 JSScript* frontend::CompileGlobalScriptWithExtraBindings(
diff --git a/js/src/jit-test/tests/basic/non-syntactic-with-unscopables.js b/js/src/jit-test/tests/basic/non-syntactic-with-unscopables.js
new file mode 100644
index 000000000000..826c7a909cf9
--- /dev/null
+++ b/js/src/jit-test/tests/basic/non-syntactic-with-unscopables.js
@@ -0,0 +1,15 @@
+// Tests evaluate's supportUnscopables option.
+function test(supportUnscopables) {
+    var env = {x: 1, y: 2};
+    Object.defineProperty(env, Symbol.unscopables, {get: function() {
+        assertEq(supportUnscopables, true);
+        return {x: false, y: true};
+    }});
+
+    evaluate(`this.gotX = x; try { this.gotY = y; } catch {}`,
+             {envChainObject: env, supportUnscopables});
+    assertEq(env.gotX, 1);
+    assertEq(env.gotY, supportUnscopables ? undefined : 2);
+}
+test(false);
+test(true);
diff --git a/js/src/jsapi-tests/testChromeBuffer.cpp b/js/src/jsapi-tests/testChromeBuffer.cpp
index 7ffe2a8ce0d0..97631b7ec236 100644
--- a/js/src/jsapi-tests/testChromeBuffer.cpp
+++ b/js/src/jsapi-tests/testChromeBuffer.cpp
@@ -9,6 +9,7 @@
 #include "js/CallAndConstruct.h"          // JS_CallFunctionValue
 #include "js/CompilationAndEvaluation.h"  // JS::CompileFunction
 #include "js/ContextOptions.h"
+#include "js/EnvironmentChain.h"    // JS::EnvironmentChain
 #include "js/GlobalObject.h"        // JS_NewGlobalObject
 #include "js/PropertyAndElement.h"  // JS_DefineProperty
 #include "js/SourceText.h"          // JS::Source{Ownership,Text}
@@ -87,8 +88,8 @@ BEGIN_TEST(testChromeBuffer) {
       JS::CompileOptions options(cx);
       options.setFileAndLine("", 0);
 
-      JS::RootedObjectVector emptyScopeChain(cx);
-      fun = JS::CompileFunction(cx, emptyScopeChain, options, "trusted", 1,
+      JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+      fun = JS::CompileFunction(cx, emptyEnvChain, options, "trusted", 1,
                                 &paramName, srcBuf);
       CHECK(fun);
       CHECK(JS_DefineProperty(cx, trusted_glob, "trusted", fun,
@@ -118,8 +119,8 @@ BEGIN_TEST(testChromeBuffer) {
     JS::CompileOptions options(cx);
     options.setFileAndLine("", 0);
 
-    JS::RootedObjectVector emptyScopeChain(cx);
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, "untrusted", 1,
+    JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, "untrusted", 1,
                               &paramName, srcBuf);
     CHECK(fun);
     CHECK(JS_DefineProperty(cx, global, "untrusted", fun, JSPROP_ENUMERATE));
@@ -165,8 +166,8 @@ BEGIN_TEST(testChromeBuffer) {
       JS::CompileOptions options(cx);
       options.setFileAndLine("", 0);
 
-      JS::RootedObjectVector emptyScopeChain(cx);
-      fun = JS::CompileFunction(cx, emptyScopeChain, options, "trusted", 1,
+      JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+      fun = JS::CompileFunction(cx, emptyEnvChain, options, "trusted", 1,
                                 &paramName, srcBuf);
       CHECK(fun);
       CHECK(JS_DefineProperty(cx, trusted_glob, "trusted", fun,
@@ -192,8 +193,8 @@ BEGIN_TEST(testChromeBuffer) {
     JS::CompileOptions options(cx);
     options.setFileAndLine("", 0);
 
-    JS::RootedObjectVector emptyScopeChain(cx);
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, "untrusted", 1,
+    JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, "untrusted", 1,
                               &paramName, srcBuf);
     CHECK(fun);
     CHECK(JS_DefineProperty(cx, global, "untrusted", fun, JSPROP_ENUMERATE));
@@ -226,8 +227,8 @@ BEGIN_TEST(testChromeBuffer) {
       JS::CompileOptions options(cx);
       options.setFileAndLine("", 0);
 
-      JS::RootedObjectVector emptyScopeChain(cx);
-      fun = JS::CompileFunction(cx, emptyScopeChain, options, "trusted", 0,
+      JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+      fun = JS::CompileFunction(cx, emptyEnvChain, options, "trusted", 0,
                                 nullptr, srcBuf);
       CHECK(fun);
       CHECK(JS_DefineProperty(cx, trusted_glob, "trusted", fun,
@@ -254,8 +255,8 @@ BEGIN_TEST(testChromeBuffer) {
     JS::CompileOptions options(cx);
     options.setFileAndLine("", 0);
 
-    JS::RootedObjectVector emptyScopeChain(cx);
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, "untrusted", 1,
+    JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, "untrusted", 1,
                               &paramName, srcBuf);
     CHECK(fun);
     CHECK(JS_DefineProperty(cx, global, "untrusted", fun, JSPROP_ENUMERATE));
diff --git a/js/src/jsapi-tests/testFunctionBinding.cpp b/js/src/jsapi-tests/testFunctionBinding.cpp
index 6a69e3234914..7f5924a0a223 100644
--- a/js/src/jsapi-tests/testFunctionBinding.cpp
+++ b/js/src/jsapi-tests/testFunctionBinding.cpp
@@ -11,6 +11,7 @@
 
 #include "js/CallAndConstruct.h"
 #include "js/CompilationAndEvaluation.h"  // JS::CompileFunction
+#include "js/EnvironmentChain.h"          // JS::EnvironmentChain
 #include "js/SourceText.h"                // JS::Source{Ownership,Text}
 #include "jsapi-tests/tests.h"
 #include "util/Text.h"
@@ -23,7 +24,7 @@ BEGIN_TEST(test_functionBinding) {
   JS::CompileOptions options(cx);
   options.setFileAndLine(__FILE__, __LINE__);
 
-  JS::RootedObjectVector emptyScopeChain(cx);
+  JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
 
   // Named function shouldn't have it's binding.
   {
@@ -33,7 +34,7 @@ BEGIN_TEST(test_functionBinding) {
     CHECK(srcBuf.init(cx, s1chars, js_strlen(s1chars),
                       JS::SourceOwnership::Borrowed));
 
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, "s1", 0, nullptr,
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, "s1", 0, nullptr,
                               srcBuf);
     CHECK(fun);
   }
@@ -52,7 +53,7 @@ BEGIN_TEST(test_functionBinding) {
     CHECK(srcBuf.init(cx, s2chars, js_strlen(s2chars),
                       JS::SourceOwnership::Borrowed));
 
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, "s2", 0, nullptr,
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, "s2", 0, nullptr,
                               srcBuf);
     CHECK(fun);
   }
@@ -69,7 +70,7 @@ BEGIN_TEST(test_functionBinding) {
     CHECK(srcBuf.init(cx, s3chars, js_strlen(s3chars),
                       JS::SourceOwnership::Borrowed));
 
-    fun = JS::CompileFunction(cx, emptyScopeChain, options, nullptr, 0, nullptr,
+    fun = JS::CompileFunction(cx, emptyEnvChain, options, nullptr, 0, nullptr,
                               srcBuf);
     CHECK(fun);
   }
diff --git a/js/src/jsapi-tests/testFunctionNonSyntactic.cpp b/js/src/jsapi-tests/testFunctionNonSyntactic.cpp
index b58db0889793..5b481d90ea2d 100644
--- a/js/src/jsapi-tests/testFunctionNonSyntactic.cpp
+++ b/js/src/jsapi-tests/testFunctionNonSyntactic.cpp
@@ -11,6 +11,7 @@
 
 #include "js/CallAndConstruct.h"
 #include "js/CompilationAndEvaluation.h"  // JS::CompileFunction
+#include "js/EnvironmentChain.h"          // JS::EnvironmentChain
 #include "js/PropertyAndElement.h"        // JS_DefineProperty
 #include "js/SourceText.h"                // JS::Source{Ownership,Text}
 #include "jsapi-tests/tests.h"
@@ -22,7 +23,7 @@
 using namespace js;
 
 BEGIN_TEST(testFunctionNonSyntactic) {
-  JS::RootedObjectVector scopeChain(cx);
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
 
   {
     JS::RootedObject scopeObj(cx, JS_NewPlainObject(cx));
@@ -30,7 +31,7 @@ BEGIN_TEST(testFunctionNonSyntactic) {
     JS::RootedValue val(cx);
     val.setNumber(1);
     CHECK(JS_DefineProperty(cx, scopeObj, "foo", val, JSPROP_ENUMERATE));
-    CHECK(scopeChain.append(scopeObj));
+    CHECK(envChain.append(scopeObj));
   }
 
   {
@@ -39,7 +40,7 @@ BEGIN_TEST(testFunctionNonSyntactic) {
     JS::RootedValue val(cx);
     val.setNumber(20);
     CHECK(JS_DefineProperty(cx, scopeObj, "bar", val, JSPROP_ENUMERATE));
-    CHECK(scopeChain.append(scopeObj));
+    CHECK(envChain.append(scopeObj));
   }
 
   {
@@ -50,8 +51,8 @@ BEGIN_TEST(testFunctionNonSyntactic) {
 
     JS::CompileOptions options(cx);
     options.setFileAndLine(__FILE__, __LINE__);
-    RootedFunction fun(cx, JS::CompileFunction(cx, scopeChain, options, "test",
-                                               0, nullptr, srcBuf));
+    RootedFunction fun(cx, JS::CompileFunction(cx, envChain, options, "test", 0,
+                                               nullptr, srcBuf));
     CHECK(fun);
 
     CHECK(fun->enclosingScope()->kind() == ScopeKind::NonSyntactic);
@@ -76,8 +77,8 @@ BEGIN_TEST(testFunctionNonSyntactic) {
 
     JS::CompileOptions options(cx);
     options.setFileAndLine(__FILE__, __LINE__);
-    RootedFunction fun(cx, JS::CompileFunction(cx, scopeChain, options, "test",
-                                               1, args, srcBuf));
+    RootedFunction fun(cx, JS::CompileFunction(cx, envChain, options, "test", 1,
+                                               args, srcBuf));
     CHECK(fun);
 
     CHECK(fun->enclosingScope()->kind() == ScopeKind::NonSyntactic);
diff --git a/js/src/jsapi-tests/testJSEvaluateScript.cpp b/js/src/jsapi-tests/testJSEvaluateScript.cpp
index 1dfc9ea3cb5f..72c1aa6eacc7 100644
--- a/js/src/jsapi-tests/testJSEvaluateScript.cpp
+++ b/js/src/jsapi-tests/testJSEvaluateScript.cpp
@@ -3,6 +3,7 @@
  */
 
 #include "js/CompilationAndEvaluation.h"
+#include "js/EnvironmentChain.h"    // JS::EnvironmentChain
 #include "js/PropertyAndElement.h"  // JS_AlreadyHasOwnProperty, JS_HasProperty
 #include "js/SourceText.h"
 #include "jsapi-tests/tests.h"
@@ -16,13 +17,13 @@ BEGIN_TEST(testJSEvaluateScript) {
 
   JS::RootedValue retval(cx);
   JS::CompileOptions opts(cx);
-  JS::RootedObjectVector scopeChain(cx);
-  CHECK(scopeChain.append(obj));
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
+  CHECK(envChain.append(obj));
 
   JS::SourceText<char16_t> srcBuf;
   CHECK(srcBuf.init(cx, src, js_strlen(src), JS::SourceOwnership::Borrowed));
 
-  CHECK(JS::Evaluate(cx, scopeChain, opts.setFileAndLine(__FILE__, __LINE__),
+  CHECK(JS::Evaluate(cx, envChain, opts.setFileAndLine(__FILE__, __LINE__),
                      srcBuf, &retval));
 
   bool hasProp = true;
diff --git a/js/src/jsapi-tests/testPreserveJitCode.cpp b/js/src/jsapi-tests/testPreserveJitCode.cpp
index e27e63f2d1cd..994f5bec2152 100644
--- a/js/src/jsapi-tests/testPreserveJitCode.cpp
+++ b/js/src/jsapi-tests/testPreserveJitCode.cpp
@@ -7,6 +7,7 @@
 #include "jit/Ion.h"                      // js::jit::IsIonEnabled
 #include "js/CallAndConstruct.h"          // JS::CallFunction
 #include "js/CompilationAndEvaluation.h"  // JS::CompileFunction
+#include "js/EnvironmentChain.h"          // JS::EnvironmentChain
 #include "js/GlobalObject.h"              // JS_NewGlobalObject
 #include "js/SourceText.h"                // JS::Source{Ownership,Text}
 #include "jsapi-tests/tests.h"
@@ -69,9 +70,9 @@ bool testPreserveJitCode(bool preserveJitCode, unsigned remainingIonScripts) {
   options.setFileAndLine(__FILE__, 1);
 
   JS::RootedFunction fun(cx);
-  JS::RootedObjectVector emptyScopeChain(cx);
-  fun = JS::CompileFunction(cx, emptyScopeChain, options, "f", 0, nullptr,
-                            srcBuf);
+  JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
+  fun =
+      JS::CompileFunction(cx, emptyEnvChain, options, "f", 0, nullptr, srcBuf);
   CHECK(fun);
 
   RootedValue value(cx);
diff --git a/js/src/jsapi-tests/testSourcePolicy.cpp b/js/src/jsapi-tests/testSourcePolicy.cpp
index b6f9d4a8d78d..e2e2796ef12e 100644
--- a/js/src/jsapi-tests/testSourcePolicy.cpp
+++ b/js/src/jsapi-tests/testSourcePolicy.cpp
@@ -5,6 +5,7 @@
 #include "mozilla/Utf8.h"  // mozilla::Utf8Unit
 
 #include "js/CompilationAndEvaluation.h"  // JS::CompileFunction, JS::Evaluate
+#include "js/EnvironmentChain.h"          // JS::EnvironmentChain
 #include "js/GlobalObject.h"              // JS_NewGlobalObject
 #include "js/MemoryFunctions.h"
 #include "js/SourceText.h"  // JS::Source{Ownership,Text}
@@ -41,13 +42,13 @@ BEGIN_TEST(testBug795104) {
   CHECK(JS::Evaluate(cx, opts, srcBuf, &unused));
 
   JS::RootedFunction fun(cx);
-  JS::RootedObjectVector emptyScopeChain(cx);
+  JS::EnvironmentChain emptyEnvChain(cx, JS::SupportUnscopables::No);
 
   // But when compiling a function we don't want to use no-rval
   // mode, since it's not supported for functions.
   opts.setNoScriptRval(false);
 
-  fun = JS::CompileFunction(cx, emptyScopeChain, opts, "f", 0, nullptr, srcBuf);
+  fun = JS::CompileFunction(cx, emptyEnvChain, opts, "f", 0, nullptr, srcBuf);
   CHECK(fun);
 
   JS_free(cx, s);
diff --git a/js/src/jsapi-tests/testStencil.cpp b/js/src/jsapi-tests/testStencil.cpp
index 7d6b78d7d8cd..8a71d273b9e0 100644
--- a/js/src/jsapi-tests/testStencil.cpp
+++ b/js/src/jsapi-tests/testStencil.cpp
@@ -11,6 +11,7 @@
 
 #include "frontend/CompilationStencil.h"
 #include "js/CompilationAndEvaluation.h"
+#include "js/EnvironmentChain.h"  // JS::EnvironmentChain
 #include "js/experimental/CompileScript.h"
 #include "js/experimental/JSStencil.h"
 #include "js/Modules.h"
@@ -136,11 +137,11 @@ BEGIN_TEST(testStencil_NonSyntactic) {
   CHECK(obj);
   CHECK(JS_SetProperty(cx, obj, "x", val));
 
-  JS::RootedObjectVector chain(cx);
-  CHECK(chain.append(obj));
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
+  CHECK(envChain.append(obj));
 
   JS::RootedValue rval(cx);
-  CHECK(JS_ExecuteScript(cx, chain, script, &rval));
+  CHECK(JS_ExecuteScript(cx, envChain, script, &rval));
   CHECK(rval.isNumber() && rval.toNumber() == 42);
 
   return true;
diff --git a/js/src/moz.build b/js/src/moz.build
index f586cbed1740..edbf2db6bd58 100644
--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -136,6 +136,7 @@ EXPORTS.js += [
     "../public/Conversions.h",
     "../public/Date.h",
     "../public/Debug.h",
+    "../public/EnvironmentChain.h",
     "../public/Equality.h",
     "../public/ErrorInterceptor.h",
     "../public/ErrorReport.h",
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index a8afc08cf806..063ffc5c81f0 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -124,8 +124,9 @@
 #include "js/CompilationAndEvaluation.h"
 #include "js/CompileOptions.h"  // JS::ReadOnlyCompileOptions, JS::CompileOptions, JS::OwningCompileOptions, JS::DecodeOptions, JS::InstantiateOptions
 #include "js/ContextOptions.h"  // JS::ContextOptions{,Ref}
-#include "js/Debug.h"     // JS::dbg::ShouldAvoidSideEffects, JS::ExecutionTrace
-#include "js/Equality.h"  // JS::SameValue
+#include "js/Debug.h"  // JS::dbg::ShouldAvoidSideEffects, JS::ExecutionTrace
+#include "js/EnvironmentChain.h"            // JS::EnvironmentChain
+#include "js/Equality.h"                    // JS::SameValue
 #include "js/ErrorReport.h"                 // JS::PrintError
 #include "js/Exception.h"                   // JS::StealPendingExceptionStack
 #include "js/experimental/BindingAllocs.h"  // JS_NewObjectWithGivenProtoAndUseAllocSite
@@ -2693,7 +2694,7 @@ static bool Evaluate(JSContext* cx, unsigned argc, Value* vp) {
   bool saveIncrementalBytecode = false;
   bool execute = true;
   bool assertEqBytecode = false;
-  JS::RootedObjectVector envChain(cx);
+  JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
   RootedObject callerGlobal(cx, cx->global());
 
   options.setIntroductionType("js shell evaluate")
@@ -2781,6 +2782,13 @@ static bool Evaluate(JSContext* cx, unsigned argc, Value* vp) {
       }
     }
 
+    if (!JS_GetProperty(cx, opts, "supportUnscopables", &v)) {
+      return false;
+    }
+    if (!v.isUndefined()) {
+      envChain.setSupportUnscopables(JS::SupportUnscopables(ToBoolean(v)));
+    }
+
     // We cannot load or save the bytecode if we have no object where the
     // bytecode cache is stored.
     if (loadBytecode || saveIncrementalBytecode) {
@@ -2796,7 +2804,7 @@ static bool Evaluate(JSContext* cx, unsigned argc, Value* vp) {
     // Wrap the envChainObject list into target realm.
     JSAutoRealm ar(cx, global);
     for (size_t i = 0; i < envChain.length(); ++i) {
-      if (!JS_WrapObject(cx, envChain[i])) {
+      if (!JS_WrapObject(cx, envChain.chain()[i])) {
         return false;
       }
     }
@@ -9746,6 +9754,8 @@ static const JSFunctionSpecWithHelp shell_functions[] = {
 "      envChainObject: object to put on the scope chain, with its fields added\n"
 "         as var bindings, akin to how elements are added to the environment in\n"
 "         event handlers in Gecko.\n"
+"      supportUnscopables: if true, support Symbol.unscopables lookups for\n"
+"         envChainObject, similar to (syntactic) with-statements.\n"
 ),
 
     JS_FN_HELP("run", Run, 1, 0,
diff --git a/js/src/vm/CompilationAndEvaluation.cpp b/js/src/vm/CompilationAndEvaluation.cpp
index 6ef1248b614f..24823539d084 100644
--- a/js/src/vm/CompilationAndEvaluation.cpp
+++ b/js/src/vm/CompilationAndEvaluation.cpp
@@ -23,6 +23,7 @@
 #include "frontend/Parser.h"  // frontend::Parser, frontend::ParseGoal
 #include "js/CharacterEncoding.h"  // JS::UTF8Chars, JS::ConstUTF8CharsZ, JS::UTF8CharsToNewTwoByteCharsZ
 #include "js/ColumnNumber.h"            // JS::ColumnNumberOneOrigin
+#include "js/EnvironmentChain.h"        // JS::EnvironmentChain
 #include "js/experimental/JSStencil.h"  // JS::Stencil
 #include "js/friend/ErrorMessages.h"    // js::GetErrorMessage, JSMSG_*
 #include "js/RootingAPI.h"              // JS::Rooted
@@ -299,7 +300,7 @@ class FunctionCompiler {
     return funStr_.append(srcBuf.get(), srcBuf.length());
   }
 
-  JSFunction* finish(HandleObjectVector envChain,
+  JSFunction* finish(const JS::EnvironmentChain& envChain,
                      const ReadOnlyCompileOptions& optionsArg) {
     using js::frontend::FunctionSyntaxKind;
 
@@ -381,7 +382,7 @@ class FunctionCompiler {
 };
 
 JS_PUBLIC_API JSFunction* JS::CompileFunction(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, SourceText<char16_t>& srcBuf) {
   ManualReportFrontendContext fc(cx);
@@ -397,7 +398,7 @@ JS_PUBLIC_API JSFunction* JS::CompileFunction(
 }
 
 JS_PUBLIC_API JSFunction* JS::CompileFunction(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, SourceText<Utf8Unit>& srcBuf) {
   ManualReportFrontendContext fc(cx);
@@ -413,7 +414,7 @@ JS_PUBLIC_API JSFunction* JS::CompileFunction(
 }
 
 JS_PUBLIC_API JSFunction* JS::CompileFunctionUtf8(
-    JSContext* cx, HandleObjectVector envChain,
+    JSContext* cx, const EnvironmentChain& envChain,
     const ReadOnlyCompileOptions& options, const char* name, unsigned nargs,
     const char* const* argnames, const char* bytes, size_t length) {
   SourceText<Utf8Unit> srcBuf;
@@ -495,7 +496,7 @@ MOZ_NEVER_INLINE static bool ExecuteScript(JSContext* cx, HandleObject envChain,
   return Execute(cx, script, envChain, rval);
 }
 
-static bool ExecuteScript(JSContext* cx, HandleObjectVector envChain,
+static bool ExecuteScript(JSContext* cx, const JS::EnvironmentChain& envChain,
                           HandleScript script, MutableHandleValue rval) {
   RootedObject env(cx, CreateNonSyntacticEnvironmentChain(cx, envChain));
   if (!env) {
@@ -520,13 +521,14 @@ MOZ_NEVER_INLINE JS_PUBLIC_API bool JS_ExecuteScript(JSContext* cx,
 }
 
 MOZ_NEVER_INLINE JS_PUBLIC_API bool JS_ExecuteScript(
-    JSContext* cx, HandleObjectVector envChain, HandleScript scriptArg,
+    JSContext* cx, const JS::EnvironmentChain& envChain, HandleScript scriptArg,
     MutableHandleValue rval) {
   return ExecuteScript(cx, envChain, scriptArg, rval);
 }
 
 MOZ_NEVER_INLINE JS_PUBLIC_API bool JS_ExecuteScript(
-    JSContext* cx, HandleObjectVector envChain, HandleScript scriptArg) {
+    JSContext* cx, const JS::EnvironmentChain& envChain,
+    HandleScript scriptArg) {
   RootedValue rval(cx);
   return ExecuteScript(cx, envChain, scriptArg, &rval);
 }
@@ -576,7 +578,7 @@ JS_PUBLIC_API bool JS::Evaluate(JSContext* cx,
                               srcBuf, rval);
 }
 
-JS_PUBLIC_API bool JS::Evaluate(JSContext* cx, HandleObjectVector envChain,
+JS_PUBLIC_API bool JS::Evaluate(JSContext* cx, const EnvironmentChain& envChain,
                                 const ReadOnlyCompileOptions& options,
                                 SourceText<char16_t>& srcBuf,
                                 MutableHandleValue rval) {
diff --git a/js/src/vm/EnvironmentObject.cpp b/js/src/vm/EnvironmentObject.cpp
index d5865ff6ffd6..f7d824b84cb8 100644
--- a/js/src/vm/EnvironmentObject.cpp
+++ b/js/src/vm/EnvironmentObject.cpp
@@ -10,6 +10,7 @@
 
 #include "builtin/Array.h"
 #include "builtin/ModuleObject.h"
+#include "js/EnvironmentChain.h"  // JS::EnvironmentChain
 #include "js/Exception.h"
 #include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
 #include "js/friend/StackLimits.h"    // js::AutoCheckRecursionLimit
@@ -717,10 +718,9 @@ JSObject* js::GetThisObject(JSObject* obj) {
   return obj;
 }
 
-WithEnvironmentObject* WithEnvironmentObject::create(JSContext* cx,
-                                                     HandleObject object,
-                                                     HandleObject enclosing,
-                                                     Handle<WithScope*> scope) {
+WithEnvironmentObject* WithEnvironmentObject::create(
+    JSContext* cx, HandleObject object, HandleObject enclosing,
+    Handle<WithScope*> scope, JS::SupportUnscopables supportUnscopables) {
   Rooted<SharedShape*> shape(cx,
                              EmptyEnvironmentShape<WithEnvironmentObject>(cx));
   if (!shape) {
@@ -738,17 +738,22 @@ WithEnvironmentObject* WithEnvironmentObject::create(JSContext* cx,
   obj->initReservedSlot(OBJECT_SLOT, ObjectValue(*object));
   obj->initReservedSlot(THIS_SLOT, ObjectValue(*thisObj));
   if (scope) {
-    obj->initReservedSlot(SCOPE_SLOT, PrivateGCThingValue(scope));
+    MOZ_ASSERT(supportUnscopables == JS::SupportUnscopables::Yes,
+               "with-statements must support Symbol.unscopables");
+    obj->initReservedSlot(SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT,
+                          PrivateGCThingValue(scope));
   } else {
-    obj->initReservedSlot(SCOPE_SLOT, NullValue());
+    Value v = BooleanValue(supportUnscopables == JS::SupportUnscopables::Yes);
+    obj->initReservedSlot(SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT, v);
   }
 
   return obj;
 }
 
 WithEnvironmentObject* WithEnvironmentObject::createNonSyntactic(
-    JSContext* cx, HandleObject object, HandleObject enclosing) {
-  return create(cx, object, enclosing, nullptr);
+    JSContext* cx, HandleObject object, HandleObject enclosing,
+    JS::SupportUnscopables supportUnscopables) {
+  return create(cx, object, enclosing, nullptr, supportUnscopables);
 }
 
 static inline bool IsUnscopableDotName(JSContext* cx, HandleId id) {
@@ -811,8 +816,9 @@ static bool with_LookupProperty(JSContext* cx, HandleObject obj, HandleId id,
   }
 
   if (propp->isFound()) {
-    bool scopable;
-    if (!CheckUnscopables(cx, actual, id, &scopable)) {
+    bool scopable = true;
+    if (obj->as<WithEnvironmentObject>().supportUnscopables() &&
+        !CheckUnscopables(cx, actual, id, &scopable)) {
       return false;
     }
     if (!scopable) {
@@ -840,7 +846,7 @@ static bool with_HasProperty(JSContext* cx, HandleObject obj, HandleId id,
   if (!HasProperty(cx, actual, id, foundp)) {
     return false;
   }
-  if (!*foundp) {
+  if (!*foundp || !obj->as<WithEnvironmentObject>().supportUnscopables()) {
     return true;
   }
 
@@ -938,7 +944,7 @@ const JSClass NonSyntacticVariablesObject::class_ = {
 };
 
 NonSyntacticLexicalEnvironmentObject* js::CreateNonSyntacticEnvironmentChain(
-    JSContext* cx, HandleObjectVector envChain) {
+    JSContext* cx, const JS::EnvironmentChain& envChain) {
   // Callers are responsible for segregating the NonSyntactic case from simple
   // compilation cases.
   MOZ_RELEASE_ASSERT(!envChain.empty());
@@ -2437,9 +2443,12 @@ class DebugEnvironmentProxyHandler : public NurseryAllocableProxyHandler {
 
     if (isWith) {
       size_t j = 0;
+      bool supportUnscopables =
+          env->as<WithEnvironmentObject>().supportUnscopables();
       for (size_t i = 0; i < props.length(); i++) {
-        bool inScope;
-        if (!CheckUnscopables(cx, env, props[i], &inScope)) {
+        bool inScope = true;
+        if (supportUnscopables &&
+            !CheckUnscopables(cx, env, props[i], &inScope)) {
           return false;
         }
         if (inScope) {
@@ -3429,13 +3438,14 @@ JSObject* js::GetDebugEnvironmentForGlobalLexicalEnvironment(JSContext* cx) {
 }
 
 WithEnvironmentObject* js::CreateObjectsForEnvironmentChain(
-    JSContext* cx, HandleObjectVector chain, HandleObject terminatingEnv) {
+    JSContext* cx, const JS::EnvironmentChain& chain,
+    HandleObject terminatingEnv) {
   MOZ_ASSERT(!chain.empty());
 
 #ifdef DEBUG
   for (size_t i = 0; i < chain.length(); ++i) {
-    cx->check(chain[i]);
-    MOZ_ASSERT(!chain[i]->isUnqualifiedVarObj());
+    cx->check(chain.chain()[i]);
+    MOZ_ASSERT(!chain.chain()[i]->isUnqualifiedVarObj());
   }
 #endif
 
@@ -3444,8 +3454,8 @@ WithEnvironmentObject* js::CreateObjectsForEnvironmentChain(
   Rooted<WithEnvironmentObject*> withEnv(cx);
   RootedObject enclosingEnv(cx, terminatingEnv);
   for (size_t i = chain.length(); i > 0;) {
-    withEnv =
-        WithEnvironmentObject::createNonSyntactic(cx, chain[--i], enclosingEnv);
+    withEnv = WithEnvironmentObject::createNonSyntactic(
+        cx, chain.chain()[--i], enclosingEnv, chain.supportUnscopables());
     if (!withEnv) {
       return nullptr;
     }
@@ -3470,14 +3480,24 @@ JSObject* WithEnvironmentObject::withThis() const {
 }
 
 bool WithEnvironmentObject::isSyntactic() const {
-  Value v = getReservedSlot(SCOPE_SLOT);
-  MOZ_ASSERT(v.isPrivateGCThing() || v.isNull());
+  Value v = getReservedSlot(SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT);
+  MOZ_ASSERT(v.isPrivateGCThing() || v.isBoolean());
   return v.isPrivateGCThing();
 }
 
+bool WithEnvironmentObject::supportUnscopables() const {
+  if (isSyntactic()) {
+    return true;
+  }
+  Value v = getReservedSlot(SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT);
+  MOZ_ASSERT(v.isBoolean());
+  return v.isTrue();
+}
+
 WithScope& WithEnvironmentObject::scope() const {
   MOZ_ASSERT(isSyntactic());
-  return *static_cast<WithScope*>(getReservedSlot(SCOPE_SLOT).toGCThing());
+  Value v = getReservedSlot(SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT);
+  return *static_cast<WithScope*>(v.toGCThing());
 }
 
 ModuleEnvironmentObject* js::GetModuleEnvironmentForScript(JSScript* script) {
diff --git a/js/src/vm/EnvironmentObject.h b/js/src/vm/EnvironmentObject.h
index 2e43569a43fc..d72749cbe3ad 100644
--- a/js/src/vm/EnvironmentObject.h
+++ b/js/src/vm/EnvironmentObject.h
@@ -20,6 +20,11 @@
 #include "vm/Scope.h"
 #include "vm/ScopeKind.h"  // ScopeKind
 
+namespace JS {
+class JS_PUBLIC_API EnvironmentChain;
+enum class SupportUnscopables : bool;
+};  // namespace JS
+
 namespace js {
 
 class AbstractGeneratorObject;
@@ -175,6 +180,11 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *
  *    Does not hold 'let' or 'const' bindings.
  *
+ *    The embedding can specify whether these non-syntactic WithEnvironment
+ *    objects support Symbol.unscopables similar to syntactic 'with' statements
+ *    in JS. In Firefox, we support Symbol.unscopables only for DOM event
+ *    handlers because this is required by the spec.
+ *
  * 2. NonSyntacticVariablesObject
  *
  *    When the embedding wants qualified 'var' bindings and unqualified
@@ -234,7 +244,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   GlobalLexicalEnvironmentObject[this=global]
  *       |
- *   WithEnvironmentObject wrapping target (qualified 'var's)
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping target
+ *     (qualified 'var's)
  *       |
  *   NonSyntacticLexicalEnvironmentObject[this=target] (lexical vars)
  *
@@ -263,7 +274,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   NonSyntacticLexicalEnvironmentObject[this=nsvo]
  *       |
- *   WithEnvironmentObject wrapping target (qualified 'var's)
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping target
+ *     (qualified 'var's)
  *       |
  *   NonSyntacticLexicalEnvironmentObject[this=target] (lexical vars)
  *
@@ -298,7 +310,7 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   NonSyntacticVariablesObject (qualified 'var's and unqualified names)
  *       |
- *   WithEnvironmentObject wrapping messageManager
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping messageManager
  *       |
  *   NonSyntacticLexicalEnvironmentObject[this=messageManager] (lexical vars)
  *
@@ -315,7 +327,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   GlobalLexicalEnvironmentObject[this=global]
  *       |
- *   WithEnvironmentObject wrapping messageManager (qualified 'var's)
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping messageManager
+ *     (qualified 'var's)
  *       |
  *   NonSyntacticLexicalEnvironmentObject[this=messageManager] (lexical vars)
  *
@@ -334,13 +347,13 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   GlobalLexicalEnvironmentObject[this=global]
  *       |
- *   WithEnvironmentObject wrapping eN
+ *   WithEnvironmentObject [SupportUnscopables=Yes] wrapping eN
  *       |
  *      ...
  *       |
- *   WithEnvironmentObject wrapping e1
+ *   WithEnvironmentObject [SupportUnscopables=Yes] wrapping e1
  *       |
- *   WithEnvironmentObject wrapping e0
+ *   WithEnvironmentObject [SupportUnscopables=Yes] wrapping e0
  *       |
  *   NonSyntacticLexicalEnvironmentObject [this=*unused*]
  *
@@ -355,9 +368,9 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *      ...
  *       |
- *   WithEnvironmentObject wrapping e1
+ *   WithEnvironmentObject [SupportUnscopables=Yes] wrapping e1
  *       |
- *   WithEnvironmentObject wrapping e0
+ *   WithEnvironmentObject [SupportUnscopables=Yes] wrapping e0
  *       |
  *   NonSyntacticLexicalEnvironmentObject [this=*unused*]
  *       |
@@ -383,7 +396,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   [DebugProxy] CallObject (qualified 'var's)
  *       |
- *   WithEnvironmentObject wrapping bindings (conflicting 'var's and names)
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping bindings
+ *     (conflicting 'var's and names)
  *
  * If the script has direct eval, BlockLexicalEnvironmentObject is created for
  * it:
@@ -394,7 +408,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   [DebugProxy] CallObject (qualified 'var's)
  *       |
- *   WithEnvironmentObject wrapping bindings (conflicting 'var's and names)
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping bindings
+ *     (conflicting 'var's and names)
  *       |
  *   BlockLexicalEnvironmentObject (lexical vars, and conflicting lexical vars)
  *
@@ -415,7 +430,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   GlobalLexicalEnvironmentObject[this=global] (lexical vars)
  *       |
- *   WithEnvironmentObject wrapping object with not-conflicting bindings
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping object with
+ *     not-conflicting bindings
  *
  * If `options.useInnerBindings` is true, all bindings are stored into the
  * bindings object wrapped by WithEnvironmentObject, and they shadow globals
@@ -424,7 +440,8 @@ extern PropertyName* EnvironmentCoordinateNameSlow(JSScript* script,
  *       |
  *   GlobalLexicalEnvironmentObject[this=global] (lexical vars)
  *       |
- *   WithEnvironmentObject wrapping object with all bindings
+ *   WithEnvironmentObject [SupportUnscopables=No] wrapping object with all
+ *     bindings
  *
  * NOTE: If `options.useInnerBindings` is true, and if lexical variable names
  *       conflict with the bindings object's properties, the write on them
@@ -982,13 +999,16 @@ class NonSyntacticVariablesObject : public EnvironmentObject {
 };
 
 NonSyntacticLexicalEnvironmentObject* CreateNonSyntacticEnvironmentChain(
-    JSContext* cx, JS::HandleObjectVector envChain);
+    JSContext* cx, const JS::EnvironmentChain& envChain);
 
 // With environment objects on the run-time environment chain.
 class WithEnvironmentObject : public EnvironmentObject {
   static constexpr uint32_t OBJECT_SLOT = 1;
   static constexpr uint32_t THIS_SLOT = 2;
-  static constexpr uint32_t SCOPE_SLOT = 3;
+  // For syntactic with-environments this slot stores the js::Scope*.
+  // For non-syntactic with-environments it stores a boolean indicating whether
+  // we need to look up and use Symbol.unscopables.
+  static constexpr uint32_t SCOPE_OR_SUPPORT_UNSCOPABLES_SLOT = 3;
 
  public:
   static const JSClass class_;
@@ -996,12 +1016,12 @@ class WithEnvironmentObject : public EnvironmentObject {
   static constexpr uint32_t RESERVED_SLOTS = 4;
   static constexpr ObjectFlags OBJECT_FLAGS = {};
 
-  static WithEnvironmentObject* create(JSContext* cx, HandleObject object,
-                                       HandleObject enclosing,
-                                       Handle<WithScope*> scope);
-  static WithEnvironmentObject* createNonSyntactic(JSContext* cx,
-                                                   HandleObject object,
-                                                   HandleObject enclosing);
+  static WithEnvironmentObject* create(
+      JSContext* cx, HandleObject object, HandleObject enclosing,
+      Handle<WithScope*> scope, JS::SupportUnscopables supportUnscopables);
+  static WithEnvironmentObject* createNonSyntactic(
+      JSContext* cx, HandleObject object, HandleObject enclosing,
+      JS::SupportUnscopables supportUnscopables);
 
   /* Return the 'o' in 'with (o)'. */
   JSObject& object() const;
@@ -1017,6 +1037,10 @@ class WithEnvironmentObject : public EnvironmentObject {
    */
   bool isSyntactic() const;
 
+  // Whether Symbol.unscopables must be supported for this with-environment.
+  // This always returns true for syntactic with-environments.
+  bool supportUnscopables() const;
+
   // For syntactic with environment objects, the with scope.
   WithScope& scope() const;
 
@@ -1565,7 +1589,8 @@ inline bool IsFrameInitialEnvironment(AbstractFramePtr frame,
 }
 
 WithEnvironmentObject* CreateObjectsForEnvironmentChain(
-    JSContext* cx, HandleObjectVector chain, HandleObject terminatingEnv);
+    JSContext* cx, const JS::EnvironmentChain& envChain,
+    HandleObject terminatingEnv);
 
 ModuleObject* GetModuleObjectForScript(JSScript* script);
 
diff --git a/js/src/vm/Interpreter-inl.h b/js/src/vm/Interpreter-inl.h
index 0372d78e7c0a..9a257e131eb5 100644
--- a/js/src/vm/Interpreter-inl.h
+++ b/js/src/vm/Interpreter-inl.h
@@ -97,7 +97,8 @@ inline bool FetchName(JSContext* cx, HandleObject receiver, HandleObject holder,
 
   /* Take the slow path if shape was not found in a native object. */
   if (!receiver->is<NativeObject>() || !holder->is<NativeObject>() ||
-      receiver->is<WithEnvironmentObject>()) {
+      (receiver->is<WithEnvironmentObject>() &&
+       receiver->as<WithEnvironmentObject>().supportUnscopables())) {
     Rooted<jsid> id(cx, NameToId(name));
     if (!GetProperty(cx, receiver, receiver, id, vp)) {
       return false;
@@ -108,8 +109,11 @@ inline bool FetchName(JSContext* cx, HandleObject receiver, HandleObject holder,
       /* Fast path for Object instance properties. */
       vp.set(holder->as<NativeObject>().getSlot(propInfo.slot()));
     } else {
+      // Unwrap 'with' environments for reasons given in
+      // GetNameBoundInEnvironment.
+      RootedObject normalized(cx, MaybeUnwrapWithEnvironment(receiver));
       RootedId id(cx, NameToId(name));
-      if (!NativeGetExistingProperty(cx, receiver, holder.as<NativeObject>(),
+      if (!NativeGetExistingProperty(cx, normalized, holder.as<NativeObject>(),
                                      id, propInfo, vp)) {
         return false;
       }
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 4199a135aa0e..3d124278e4a5 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -31,6 +31,7 @@
 #include "jit/BaselineJIT.h"
 #include "jit/Jit.h"
 #include "jit/JitRuntime.h"
+#include "js/EnvironmentChain.h"      // JS::SupportUnscopables
 #include "js/experimental/JitInfo.h"  // JSJitInfo
 #include "js/friend/ErrorMessages.h"  // js::GetErrorMessage, JSMSG_*
 #include "js/friend/StackLimits.h"    // js::AutoCheckRecursionLimit
@@ -1071,8 +1072,8 @@ bool js::EnterWithOperation(JSContext* cx, AbstractFramePtr frame,
   }
 
   RootedObject envChain(cx, frame.environmentChain());
-  WithEnvironmentObject* withobj =
-      WithEnvironmentObject::create(cx, obj, envChain, scope);
+  WithEnvironmentObject* withobj = WithEnvironmentObject::create(
+      cx, obj, envChain, scope, JS::SupportUnscopables::Yes);
   if (!withobj) {
     return false;
   }
diff --git a/js/xpconnect/loader/mozJSSubScriptLoader.cpp b/js/xpconnect/loader/mozJSSubScriptLoader.cpp
index a2834e1f3bc6..871a10ef8b56 100644
--- a/js/xpconnect/loader/mozJSSubScriptLoader.cpp
+++ b/js/xpconnect/loader/mozJSSubScriptLoader.cpp
@@ -21,6 +21,7 @@
 #include "xpcprivate.h"                   // xpc::OptionsBase
 #include "js/CompilationAndEvaluation.h"  // JS::Compile
 #include "js/CompileOptions.h"  // JS::ReadOnlyCompileOptions, JS::DecodeOptions
+#include "js/EnvironmentChain.h"  // JS::EnvironmentChain
 #include "js/friend/JSMEnvironment.h"  // JS::ExecuteInJSMEnvironment, JS::IsJSMEnvironment
 #include "js/SourceText.h"             // JS::Source{Ownership,Text}
 #include "js/Wrapper.h"
@@ -146,7 +147,7 @@ static bool EvalStencil(JSContext* cx, HandleObject targetObj,
     }
     retval.setUndefined();
   } else {
-    JS::RootedObjectVector envChain(cx);
+    JS::EnvironmentChain envChain(cx, JS::SupportUnscopables::No);
     if (!envChain.append(targetObj)) {
       return false;
     }