diff --git a/dom/base/CCGCScheduler.h b/dom/base/CCGCScheduler.h
index 09e992bce606..e61fbdc70e16 100644
--- a/dom/base/CCGCScheduler.h
+++ b/dom/base/CCGCScheduler.h
@@ -167,8 +167,14 @@ class CCGCScheduler {
     mNeedsGCAfterCC = true;
   }
 
-  void NoteReadyForMajorGC() {
+  // Returns false if we started and finished a major GC while waiting for a
+  // response.
+  [[nodiscard]] bool NoteReadyForMajorGC() {
+    if (mMajorGCReason == JS::GCReason::NO_REASON) {
+      return false;
+    }
     mReadyForMajorGC = true;
+    return true;
   }
 
   void NoteGCBegin() {
@@ -681,6 +687,8 @@ CCRunnerStep CCGCScheduler::AdvanceCCRunner(TimeStamp aDeadline) {
 }
 
 GCRunnerStep CCGCScheduler::GetNextGCRunnerAction(TimeStamp aDeadline) {
+  MOZ_ASSERT(mMajorGCReason != JS::GCReason::NO_REASON);
+
   if (InIncrementalGC()) {
     return {GCRunnerAction::GCSlice, mMajorGCReason};
   }
diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index 3707c5b6bc63..f970e63171df 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1606,8 +1606,7 @@ bool GCRunnerFired(TimeStamp aDeadline, void* aClosure) {
   GCRunnerStep step = sScheduler.GetNextGCRunnerAction(aDeadline);
   switch (step.mAction) {
     case GCRunnerAction::None:
-      nsJSContext::KillGCRunner();
-      return false;
+      MOZ_CRASH("Unexpected GCRunnerAction");
 
     case GCRunnerAction::WaitToMajorGC: {
       RefPtr<MayGCPromise> mbPromise = MayGCNow(step.mReason);
@@ -1621,7 +1620,9 @@ bool GCRunnerFired(TimeStamp aDeadline, void* aClosure) {
       mbPromise->Then(
           GetMainThreadSerialEventTarget(), __func__,
           [](bool aIgnored) {
-            sScheduler.NoteReadyForMajorGC();
+            if (!sScheduler.NoteReadyForMajorGC()) {
+              return;  // Another GC completed while waiting.
+            }
             // If a new runner was started, recreate it with a 0 delay. The new
             // runner will continue in idle time.
             nsJSContext::KillGCRunner();