--- a/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
+++ b/toolkit/components/url-classifier/nsUrlClassifierDBService.cpp
@@ -1887,16 +1887,21 @@ nsUrlClassifierDBService::Observe(nsISup
   } else if (!strcmp(aTopic, "quit-application")) {
     Shutdown();
   } else if (!strcmp(aTopic, "profile-before-change")) {
     // Unit test does not receive "quit-application",
     // need call shutdown in this case
     Shutdown();
     LOG(("joining background thread"));
     mWorkerProxy = nullptr;
+
+    if (!gDbBackgroundThread) {
+      return NS_OK;
+    }
+
     nsIThread *backgroundThread = gDbBackgroundThread;
     gDbBackgroundThread = nullptr;
     backgroundThread->Shutdown();
     NS_RELEASE(backgroundThread);
   } else {
     return NS_ERROR_UNEXPECTED;
   }