diff --git a/caps/BasePrincipal.cpp b/caps/BasePrincipal.cpp
index 6cd94741b0a9..a805b6325c1d 100644
--- a/caps/BasePrincipal.cpp
+++ b/caps/BasePrincipal.cpp
@@ -1190,13 +1190,6 @@ BasePrincipal::GetPrivateBrowsingId(uint32_t* aPrivateBrowsingId) {
   return NS_OK;
 }
 
-NS_IMETHODIMP
-BasePrincipal::GetIsInIsolatedMozBrowserElement(
-    bool* aIsInIsolatedMozBrowserElement) {
-  *aIsInIsolatedMozBrowserElement = IsInIsolatedMozBrowserElement();
-  return NS_OK;
-}
-
 nsresult BasePrincipal::GetAddonPolicy(
     extensions::WebExtensionPolicy** aResult) {
   AssertIsOnMainThread();
diff --git a/caps/BasePrincipal.h b/caps/BasePrincipal.h
index 9304be7dee3f..c1886646122d 100644
--- a/caps/BasePrincipal.h
+++ b/caps/BasePrincipal.h
@@ -163,8 +163,6 @@ class BasePrincipal : public nsJSPrincipals {
   NS_IMETHOD GetIsIpAddress(bool* aIsIpAddress) override;
   NS_IMETHOD GetIsLocalIpAddress(bool* aIsIpAddress) override;
   NS_IMETHOD GetIsOnion(bool* aIsOnion) override;
-  NS_IMETHOD GetIsInIsolatedMozBrowserElement(
-      bool* aIsInIsolatedMozBrowserElement) final;
   NS_IMETHOD GetUserContextId(uint32_t* aUserContextId) final;
   NS_IMETHOD GetPrivateBrowsingId(uint32_t* aPrivateBrowsingId) final;
   NS_IMETHOD GetSiteOrigin(nsACString& aSiteOrigin) final;
@@ -250,9 +248,6 @@ class BasePrincipal : public nsJSPrincipals {
   uint32_t PrivateBrowsingId() const {
     return mOriginAttributes.mPrivateBrowsingId;
   }
-  bool IsInIsolatedMozBrowserElement() const {
-    return mOriginAttributes.mInIsolatedMozBrowser;
-  }
 
   PrincipalKind Kind() const { return mKind; }
 
diff --git a/caps/nsIPrincipal.idl b/caps/nsIPrincipal.idl
index bf60f83b0468..a3849dd72315 100644
--- a/caps/nsIPrincipal.idl
+++ b/caps/nsIPrincipal.idl
@@ -586,17 +586,6 @@ interface nsIPrincipal : nsISupports
      */
     [infallible] readonly attribute unsigned long privateBrowsingId;
 
-    /**
-     * Returns true iff the principal is inside an isolated mozbrowser element.
-     * <xul:browser> is not considered to be a mozbrowser element.
-     * <iframe mozbrowser noisolation> does not count as isolated since
-     * isolation is disabled.  Isolation can only be disabled if the
-     * containing document is chrome.
-     *
-     * May be called from any thread.
-     */
-    [infallible] readonly attribute boolean isInIsolatedMozBrowserElement;
-
     /**
      * Returns true iff this is a null principal (corresponding to an
      * unknown, hence assumed minimally privileged, security context).
diff --git a/extensions/permissions/PermissionManager.cpp b/extensions/permissions/PermissionManager.cpp
index 4dd0c1acf919..cec2275f4901 100644
--- a/extensions/permissions/PermissionManager.cpp
+++ b/extensions/permissions/PermissionManager.cpp
@@ -308,17 +308,6 @@ nsresult GetPrincipalFromOrigin(const nsACString& aOrigin, bool aForceStripOA,
   return NS_OK;
 }
 
-nsresult GetPrincipal(nsIURI* aURI, bool aIsInIsolatedMozBrowserElement,
-                      nsIPrincipal** aPrincipal) {
-  OriginAttributes attrs(aIsInIsolatedMozBrowserElement);
-  nsCOMPtr<nsIPrincipal> principal =
-      BasePrincipal::CreateContentPrincipal(aURI, attrs);
-  NS_ENSURE_TRUE(principal, NS_ERROR_FAILURE);
-
-  principal.forget(aPrincipal);
-  return NS_OK;
-}
-
 nsresult GetPrincipal(nsIURI* aURI, nsIPrincipal** aPrincipal) {
   OriginAttributes attrs;
   nsCOMPtr<nsIPrincipal> principal =
@@ -369,7 +358,6 @@ already_AddRefed<nsIURI> GetNextSubDomainURI(nsIURI* aURI) {
 nsresult UpgradeHostToOriginAndInsert(
     const nsACString& aHost, const nsCString& aType, uint32_t aPermission,
     uint32_t aExpireType, int64_t aExpireTime, int64_t aModificationTime,
-    bool aIsInIsolatedMozBrowserElement,
     std::function<nsresult(const nsACString& aOrigin, const nsCString& aType,
                            uint32_t aPermission, uint32_t aExpireType,
                            int64_t aExpireTime, int64_t aModificationTime)>&&
@@ -396,8 +384,7 @@ nsresult UpgradeHostToOriginAndInsert(
     }
 
     nsCOMPtr<nsIPrincipal> principal;
-    rv = GetPrincipal(uri, aIsInIsolatedMozBrowserElement,
-                      getter_AddRefs(principal));
+    rv = GetPrincipal(uri, getter_AddRefs(principal));
     NS_ENSURE_SUCCESS(rv, rv);
 
     nsAutoCString origin;
@@ -512,8 +499,7 @@ nsresult UpgradeHostToOriginAndInsert(
 
       // We now have a URI which we can make a nsIPrincipal out of
       nsCOMPtr<nsIPrincipal> principal;
-      rv = GetPrincipal(uri, aIsInIsolatedMozBrowserElement,
-                        getter_AddRefs(principal));
+      rv = GetPrincipal(uri, getter_AddRefs(principal));
       if (NS_WARN_IF(NS_FAILED(rv))) continue;
 
       nsAutoCString origin;
@@ -560,8 +546,7 @@ nsresult UpgradeHostToOriginAndInsert(
     rv = NS_NewURI(getter_AddRefs(uri), "http://"_ns + hostSegment);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    rv = GetPrincipal(uri, aIsInIsolatedMozBrowserElement,
-                      getter_AddRefs(principal));
+    rv = GetPrincipal(uri, getter_AddRefs(principal));
     NS_ENSURE_SUCCESS(rv, rv);
 
     rv = GetOriginFromPrincipal(principal, IsOAForceStripPermission(aType),
@@ -575,8 +560,7 @@ nsresult UpgradeHostToOriginAndInsert(
     rv = NS_NewURI(getter_AddRefs(uri), "https://"_ns + hostSegment);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    rv = GetPrincipal(uri, aIsInIsolatedMozBrowserElement,
-                      getter_AddRefs(principal));
+    rv = GetPrincipal(uri, getter_AddRefs(principal));
     NS_ENSURE_SUCCESS(rv, rv);
 
     rv = GetOriginFromPrincipal(principal, IsOAForceStripPermission(aType),
@@ -3068,7 +3052,7 @@ void PermissionManager::CompleteMigrations() {
   for (const MigrationEntry& entry : entries) {
     rv = UpgradeHostToOriginAndInsert(
         entry.mHost, entry.mType, entry.mPermission, entry.mExpireType,
-        entry.mExpireTime, entry.mModificationTime, entry.mIsInBrowserElement,
+        entry.mExpireTime, entry.mModificationTime,
         [&](const nsACString& aOrigin, const nsCString& aType,
             uint32_t aPermission, uint32_t aExpireType, int64_t aExpireTime,
             int64_t aModificationTime) {
@@ -3689,7 +3673,7 @@ nsresult PermissionManager::ImportLatestDefaults() {
 
       rv = UpgradeHostToOriginAndInsert(
           entry.mHostOrOrigin, entry.mType, entry.mPermission,
-          nsIPermissionManager::EXPIRE_NEVER, 0, modificationTime, false,
+          nsIPermissionManager::EXPIRE_NEVER, 0, modificationTime,
           [&](const nsACString& aOrigin, const nsCString& aType,
               uint32_t aPermission, uint32_t aExpireType, int64_t aExpireTime,
               int64_t aModificationTime) {
diff --git a/netwerk/test/unit/test_cache_jar.js b/netwerk/test/unit/test_cache_jar.js
index bedde668b481..1ab93980a60c 100644
--- a/netwerk/test/unit/test_cache_jar.js
+++ b/netwerk/test/unit/test_cache_jar.js
@@ -20,27 +20,24 @@ function cached_handler(metadata, response) {
   handlers_called++;
 }
 
-function makeChan(url, inIsolatedMozBrowser, userContextId) {
+function makeChan(url, userContextId) {
   var chan = NetUtil.newChannel({
     uri: url,
     loadUsingSystemPrincipal: true,
   }).QueryInterface(Ci.nsIHttpChannel);
-  chan.loadInfo.originAttributes = { inIsolatedMozBrowser, userContextId };
+  chan.loadInfo.originAttributes = { userContextId };
   return chan;
 }
 
-// [inIsolatedMozBrowser, userContextId, expected_handlers_called]
+// [userContextId, expected_handlers_called]
 var firstTests = [
-  [false, 0, 1],
-  [true, 0, 1],
-  [false, 1, 1],
-  [true, 1, 1],
+  [0, 1],
+  [1, 1],
 ];
 var secondTests = [
-  [false, 0, 0],
-  [true, 0, 0],
-  [false, 1, 1],
-  [true, 1, 0],
+  [0, 0],
+  [1, 1],
+  [1, 0],
 ];
 
 async function run_all_tests() {
@@ -81,9 +78,9 @@ function run_test() {
   });
 }
 
-function test_channel(inIsolatedMozBrowser, userContextId, expected) {
+function test_channel(userContextId, expected) {
   return new Promise(resolve => {
-    var chan = makeChan(URL, inIsolatedMozBrowser, userContextId);
+    var chan = makeChan(URL, userContextId);
     chan.asyncOpen(
       new ChannelListener(doneFirstLoad.bind(null, resolve), expected)
     );
@@ -93,7 +90,7 @@ function test_channel(inIsolatedMozBrowser, userContextId, expected) {
 function doneFirstLoad(resolve, req, buffer, expected) {
   // Load it again, make sure it hits the cache
   var oa = req.loadInfo.originAttributes;
-  var chan = makeChan(URL, oa.isInIsolatedMozBrowserElement, oa.userContextId);
+  var chan = makeChan(URL, oa.userContextId);
   chan.asyncOpen(
     new ChannelListener(doneSecondLoad.bind(null, resolve), expected)
   );