fix: use background.scripts for Firefox MV3 compatibility

background.js

@@ -1,7 +1,5 @@
 'use strict';
 
-importScripts('libs/pdf-lib.min.js', 'parsers.js', 'pdf.js');
-
 function errMsg(err) {
   return String((err && err.message) || err);
 }

manifest.json

@@ -6,7 +6,7 @@
   "permissions": ["storage", "downloads", "activeTab"],
   "host_permissions": ["https://ismis.usc.edu.ph/*"],
   "background": {
-    "service_worker": "background.js"
+    "scripts": ["libs/pdf-lib.min.js", "parsers.js", "pdf.js", "background.js"]
   },
   "content_scripts": [
     {