--- a/toolkit/components/extensions/webrequest/StreamFilterParent.cpp
+++ b/toolkit/components/extensions/webrequest/StreamFilterParent.cpp
@@ -413,18 +413,21 @@ StreamFilterParent::OnStartRequest(nsIRe
     });
   }
 
   nsresult rv = mOrigListener->OnStartRequest(aRequest, aContext);
 
   // Important: Do this only *after* running the next listener in the chain, so
   // that we get the final delivery target after any retargeting that it may do.
   if (nsCOMPtr<nsIThreadRetargetableRequest> req = do_QueryInterface(aRequest)) {
-    Unused << req->GetDeliveryTarget(getter_AddRefs(mIOThread));
-    MOZ_ASSERT(mIOThread);
+    nsCOMPtr<nsIEventTarget> thread;
+    Unused << req->GetDeliveryTarget(getter_AddRefs(thread));
+    if (thread) {
+      mIOThread = Move(thread);
+    }
   }
 
   return rv;
 }
 
 NS_IMETHODIMP
 StreamFilterParent::OnStopRequest(nsIRequest* aRequest,
                                   nsISupports* aContext,