langpack-fix.diff

diff --git a/toolkit/components/extensions/Extension.jsm b/toolkit/components/extensions/Extension.jsm
--- a/toolkit/components/extensions/Extension.jsm
+++ b/toolkit/components/extensions/Extension.jsm
@@ -2007,6 +2007,11 @@ class Langpack extends ExtensionData {
   }
 
   async startup(reason) {
+    if (reason === "APP_STARTUP" && AppConstants.platform === "XP_WIN") {
+      if (!(await OS.File.exists(this.rootURI))) {
+        return false;
+      }
+    }
     this.chromeRegistryHandle = null;
     if (this.startupData.chromeEntries.length > 0) {
       const manifestURI = Services.io.newURI("manifest.json", null, this.rootURI);