--- a/xpcom/components/nsComponentManager.cpp
+++ b/xpcom/components/nsComponentManager.cpp
@@ -1744,17 +1744,22 @@ nsComponentManagerImpl::IsContractIDRegi
 {
   if (NS_WARN_IF(!aClass)) {
     return NS_ERROR_INVALID_ARG;
   }
 
   nsFactoryEntry* entry = GetFactoryEntry(aClass, strlen(aClass));
 
   if (entry) {
-    *aResult = true;
+    // UnregisterFactory might have left a stale nsFactoryEntry in
+    // mContractIDs, so we should check to see whether this entry has
+    // anything useful.
+    *aResult = (bool(entry->mModule) ||
+                bool(entry->mFactory) ||
+                bool(entry->mServiceObject));
   } else {
     *aResult = false;
   }
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsComponentManagerImpl::EnumerateCIDs(nsISimpleEnumerator** aEnumerator)