--- a/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini
+++ b/toolkit/components/extensions/test/xpcshell/xpcshell-common.ini
@@ -77,17 +77,16 @@ skip-if = os == "android"
 [test_ext_storage_sync_crypto.js]
 skip-if = os == "android"
 [test_ext_storage_telemetry.js]
 skip-if = os == "android" # checking for telemetry needs to be updated: 1384923
 [test_ext_trustworthy_origin.js]
 [test_ext_topSites.js]
 skip-if = os == "android"
 [test_ext_webRequest_filterResponseData.js]
-skip-if = debug
 [test_ext_webRequest_permission.js]
 [test_ext_webRequest_responseBody.js]
 [test_ext_webRequest_set_cookie.js]
 [test_ext_webRequest_suspend.js]
 [test_ext_webRequest_webSocket.js]
 [test_native_manifests.js]
 subprocess = true
 skip-if = os == "android"

--- a/toolkit/components/extensions/webrequest/StreamFilterParent.cpp
+++ b/toolkit/components/extensions/webrequest/StreamFilterParent.cpp
@@ -367,17 +367,16 @@ StreamFilterParent::RecvFlushedData()
  * Data output
  *****************************************************************************/
 
 IPCResult
 StreamFilterParent::RecvWrite(Data&& aData)
 {
   AssertIsActorThread();
 
-
   RunOnIOThread(
     NewRunnableMethod<Data&&>("StreamFilterParent::WriteMove",
                               this,
                               &StreamFilterParent::WriteMove,
                               Move(aData)));
   return IPC_OK();
 }
 
@@ -425,38 +424,42 @@ StreamFilterParent::OnStartRequest(nsIRe
     RunOnActorThread(FUNC, [=] {
       if (self->IPCActive()) {
         self->mState = State::Disconnected;
         CheckResult(self->SendError(NS_LITERAL_CSTRING("Channel redirected")));
       }
     });
   }
 
-  if (!mDisconnected) {
-    RefPtr<StreamFilterParent> self(this);
-    RunOnActorThread(FUNC, [=] {
-      if (self->IPCActive()) {
-        self->mState = State::TransferringData;
-        self->CheckResult(self->SendStartRequest());
-      }
-    });
-  }
-
   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)) {
     nsCOMPtr<nsIEventTarget> thread;
     Unused << req->GetDeliveryTarget(getter_AddRefs(thread));
     if (thread) {
       mIOThread = Move(thread);
     }
   }
 
+  // Important: Do this *after* we have set the thread delivery target, or it is
+  // possible in rare circumstances for an extension to attempt to write data
+  // before the thread has been set up, even though there are several layers of
+  // asynchrony involved.
+  if (!mDisconnected) {
+    RefPtr<StreamFilterParent> self(this);
+    RunOnActorThread(FUNC, [=] {
+      if (self->IPCActive()) {
+        self->mState = State::TransferringData;
+        self->CheckResult(self->SendStartRequest());
+      }
+    });
+  }
+
   return rv;
 }
 
 NS_IMETHODIMP
 StreamFilterParent::OnStopRequest(nsIRequest* aRequest,
                                   nsISupports* aContext,
                                   nsresult aStatusCode)
 {