--- a/xpcom/base/nsCycleCollector.cpp
+++ b/xpcom/base/nsCycleCollector.cpp
@@ -3606,17 +3606,18 @@ nsCycleCollector::FixGrayBits(bool aForc
     mCCJSRuntime->GarbageCollect(aForceGC ? JS::gcreason::SHUTDOWN_CC :
                                           JS::gcreason::CC_FORCED);
 
     mCCJSRuntime->FixWeakMappingGrayBits();
 
     // It's possible that FixWeakMappingGrayBits will hit OOM when unmarking
     // gray and we will have to go round again. The second time there should not
     // be any weak mappings to fix up so the loop body should run at most twice.
-    MOZ_RELEASE_ASSERT(count++ < 2);
+    MOZ_RELEASE_ASSERT(count < 2);
+    count++;
   } while (!mCCJSRuntime->AreGCGrayBitsValid());
 
   aTimeLog.Checkpoint("FixGrayBits");
 }
 
 bool
 nsCycleCollector::IsIncrementalGCInProgress()
 {