--- a/dom/workers/ScriptLoader.cpp
+++ b/dom/workers/ScriptLoader.cpp
@@ -889,20 +889,29 @@ private:
     return NS_OK;
   }
 
   nsresult
   LoadScript(uint32_t aIndex)
   {
     AssertIsOnMainThread();
     MOZ_ASSERT(aIndex < mLoadInfos.Length());
+    MOZ_ASSERT_IF(IsMainWorkerScript(), mWorkerScriptType != DebuggerScript);
 
     WorkerPrivate* parentWorker = mWorkerPrivate->GetParent();
 
-    nsIPrincipal* principal = mWorkerPrivate->GetPrincipal();
+    // For JavaScript debugging, the devtools server must run on the same
+    // thread as the debuggee, indicating the worker uses content principal.
+    // However, in Bug 863246, web content will no longer be able to load
+    // resource:// URIs by default, so we need system principal to load
+    // debugger scripts.
+    nsIPrincipal* principal = (mWorkerScriptType == DebuggerScript) ?
+                              nsContentUtils::GetSystemPrincipal() :
+                              mWorkerPrivate->GetPrincipal();
+
     nsCOMPtr<nsILoadGroup> loadGroup = mWorkerPrivate->GetLoadGroup();
     MOZ_DIAGNOSTIC_ASSERT(principal);
 
     NS_ENSURE_TRUE(NS_LoadGroupMatchesPrincipal(loadGroup, principal),
                    NS_ERROR_FAILURE);
 
     // Figure out our base URI.
     nsCOMPtr<nsIURI> baseURI = GetBaseURI(mIsMainScript, mWorkerPrivate);