From 619924231781b871228b21137af1a56e4e21302b Mon Sep 17 00:00:00 2001
From: ownedbywuigi <ownedbywuigi@gmail.com>
Date: Sun, 29 Mar 2026 21:45:49 +0100
Subject: [PATCH] Attempt to fix
 https://forum.palemoon.org/viewtopic.php?f=3&t=33260

---
 dom/xhr/XMLHttpRequestMainThread.cpp | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/dom/xhr/XMLHttpRequestMainThread.cpp b/dom/xhr/XMLHttpRequestMainThread.cpp
index c95c6160fc..dab0248fcc 100644
--- a/dom/xhr/XMLHttpRequestMainThread.cpp
+++ b/dom/xhr/XMLHttpRequestMainThread.cpp
@@ -3543,6 +3543,13 @@ XMLHttpRequestMainThread::ShouldBlockAuthPrompt()
   // Verify that it's ok to prompt for credentials here, per spec
   // http://xhr.spec.whatwg.org/#the-send%28%29-method
 
+  // Privileged add-ons can still issue synchronous XHR from chrome context.
+  // Showing a blocking HTTP auth dialog in that path can deadlock the UI when
+  // extension code is already running inside a nested event loop.
+  if (mFlagSynchronous && IsSystemXHR()) {
+    return true;
+  }
+
   if (mAuthorRequestHeaders.Has("authorization")) {
     return true;
   }