diff --git a/js/xpconnect/src/XPCShellImpl.cpp b/js/xpconnect/src/XPCShellImpl.cpp
index dff07aa3f8c0..24b89ce1aa9a 100644
--- a/js/xpconnect/src/XPCShellImpl.cpp
+++ b/js/xpconnect/src/XPCShellImpl.cpp
@@ -1063,14 +1063,16 @@ int XRE_XPCShellMain(int argc, char** argv, char** envp,
   PR_SetEnv("MOZ_DISABLE_ASAN_REPORTER=1");
 #endif
 
-  if (PR_GetEnv("MOZ_CHAOSMODE")) {
+  if (auto* featureStr = PR_GetEnv("MOZ_CHAOSMODE")) {
     ChaosFeature feature = ChaosFeature::Any;
-    long featureInt = strtol(PR_GetEnv("MOZ_CHAOSMODE"), nullptr, 16);
+    long featureInt = strtol(featureStr, nullptr, 16);
     if (featureInt) {
       // NOTE: MOZ_CHAOSMODE=0 or a non-hex value maps to Any feature.
       feature = static_cast<ChaosFeature>(featureInt);
     }
     ChaosMode::SetChaosFeature(feature);
+    ChaosMode::enterChaosMode();
+    MOZ_ASSERT(ChaosMode::isActive(ChaosFeature::Any));
   }
 
   if (ChaosMode::isActive(ChaosFeature::Any)) {
diff --git a/mfbt/ChaosMode.cpp b/mfbt/ChaosMode.cpp
index d090e8a37e92..b2a6fd40d440 100644
--- a/mfbt/ChaosMode.cpp
+++ b/mfbt/ChaosMode.cpp
@@ -10,8 +10,8 @@ namespace mozilla {
 
 namespace detail {
 
-Atomic<uint32_t, SequentiallyConsistent> gChaosModeCounter(0);
-ChaosFeature gChaosFeatures = None;
+Atomic<uint32_t, Relaxed> gChaosModeCounter(0);
+ChaosFeature gChaosFeatures = ChaosFeature::Any;
 
 } /* namespace detail */
 } /* namespace mozilla */
diff --git a/mfbt/ChaosMode.h b/mfbt/ChaosMode.h
index faf7acddf391..145a75e38000 100644
--- a/mfbt/ChaosMode.h
+++ b/mfbt/ChaosMode.h
@@ -37,7 +37,7 @@ enum ChaosFeature {
 };
 
 namespace detail {
-extern MFBT_DATA Atomic<uint32_t, SequentiallyConsistent> gChaosModeCounter;
+extern MFBT_DATA Atomic<uint32_t, Relaxed> gChaosModeCounter;
 extern MFBT_DATA ChaosFeature gChaosFeatures;
 }  // namespace detail
 
@@ -53,17 +53,13 @@ class ChaosMode {
   }
 
   static bool isActive(ChaosFeature aFeature) {
-    if (detail::gChaosModeCounter > 0) {
-      return true;
-    }
-    return detail::gChaosFeatures & aFeature;
+    return detail::gChaosModeCounter > 0 && (detail::gChaosFeatures & aFeature);
   }
 
   /**
    * Increase the chaos mode activation level. An equivalent number of
    * calls to leaveChaosMode must be made in order to restore the original
-   * chaos mode state. If the activation level is nonzero all chaos mode
-   * features are activated.
+   * chaos mode state.
    */
   static void enterChaosMode() { detail::gChaosModeCounter++; }
 
diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index dd2021fbe478..f2b5d666fe23 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -3999,14 +3999,16 @@ int XREMain::XRE_mainInit(bool* aExitFlag) {
   }
 #endif  // ANDROID
 
-  if (PR_GetEnv("MOZ_CHAOSMODE")) {
+  if (auto* featureStr = PR_GetEnv("MOZ_CHAOSMODE")) {
     ChaosFeature feature = ChaosFeature::Any;
-    long featureInt = strtol(PR_GetEnv("MOZ_CHAOSMODE"), nullptr, 16);
+    long featureInt = strtol(featureStr, nullptr, 16);
     if (featureInt) {
       // NOTE: MOZ_CHAOSMODE=0 or a non-hex value maps to Any feature.
       feature = static_cast<ChaosFeature>(featureInt);
     }
     ChaosMode::SetChaosFeature(feature);
+    ChaosMode::enterChaosMode();
+    MOZ_ASSERT(ChaosMode::isActive(ChaosFeature::Any));
   }
 
   if (CheckArgExists("fxr")) {