--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1780,20 +1780,18 @@ nsJSContext::BeginCycleCollectionCallbac
   gCCStats.mSuspected = nsCycleCollector_suspectedCount();
 
   KillCCTimer();
 
   gCCStats.RunForgetSkippable();
 
   MOZ_ASSERT(!sICCTimer, "Tried to create a new ICC timer when one already existed.");
 
-  if (!sIncrementalCC) {
-    return;
-  }
-
+  // Create an ICC timer even if ICC is globally disabled, because we could be manually triggering
+  // an incremental collection, and we want to be sure to finish it.
   CallCreateInstance("@mozilla.org/timer;1", &sICCTimer);
   if (sICCTimer) {
     sICCTimer->InitWithFuncCallback(ICCTimerFired,
                                     nullptr,
                                     kICCIntersliceDelay,
                                     nsITimer::TYPE_REPEATING_SLACK);
   }
 }