diff --git a/src/UserJS/main.js b/src/UserJS/main.js
index c0e4661..c2597b7 100644
--- a/src/UserJS/main.js
+++ b/src/UserJS/main.js
@@ -424,9 +424,10 @@ class i18nHandler {
    */
   i18n$(key) {
     const { navigator } = _self;
-    const current = navigator.language.split('-')[0] ?? 'en';
+    const lang = navigator.language.replace('-', '_');
+    const current = lang.split('_')[0] ?? 'en';
     try {
-      return i18nMap.get(current)?.[key] ?? 'Invalid Key';
+      return i18nMap.get(lang)?.[key] ?? i18nMap.get(current)?.[key] ?? i18nMap.get('en')?.[key] ?? 'Invalid Key';
     } catch (e) {
       err(e);
       return 'error';
@@ -435,7 +436,9 @@ class i18nHandler {
 
   get current() {
     const { navigator } = _self;
-    return navigator.language.split('-')[0] ?? 'en';
+    const lang = navigator.language.replace('-', '_');
+    const current = lang.split('_')[0] ?? 'en';
+    return i18nMap.has(lang) ? lang : i18nMap.has(current) ? current : 'en';
   }
 }
 const language = new i18nHandler();
diff --git a/src/js/ext.js b/src/js/ext.js
index ebd6ec7..986623f 100644
--- a/src/js/ext.js
+++ b/src/js/ext.js
@@ -1,5 +1,5 @@
 export const webext =
-  self.browser instanceof Object && self.browser instanceof Element === false
+  self.browser instanceof Object && (typeof Element === 'undefined' || !(self.browser instanceof Element))
     ? self.browser
     : self.chrome;
 export const runtime = webext.runtime;
diff --git a/src/js/i18n.js b/src/js/i18n.js
index c8a39a0..935bdc3 100644
--- a/src/js/i18n.js
+++ b/src/js/i18n.js
@@ -1,7 +1,7 @@
 'use strict';
 
 const i18n =
-  self.browser instanceof Object && self.browser instanceof Element === false
+  self.browser instanceof Object && (typeof Element === 'undefined' || !(self.browser instanceof Element))
     ? self.browser.i18n
     : self.chrome.i18n;
 
@@ -31,12 +31,15 @@ class i18nHandler {
 const { i18n$, toDate, toNumber } = i18nHandler;
 const language = { i18n$, toDate, toNumber };
 
-const isBackgroundProcess = document && document.title === 'Magic UserJS+ Background Page';
+// In a Service Worker, `document` is not defined — treat as background process to skip DOM rendering.
+const isBackgroundProcess =
+  typeof document === 'undefined' || document.title === 'Magic UserJS+ Background Page';
 
 if (isBackgroundProcess !== true) {
   // Helper to deal with the i18n'ing of HTML files.
   i18n.render = function (context) {
-    const docu = document;
+    const docu = typeof document !== 'undefined' ? document : null;
+    if (!docu) return;
     const root = context || docu;
     for (const elem of root.querySelectorAll('[data-i18n]')) {
       const text = i18n$(elem.getAttribute('data-i18n'));
diff --git a/src/js/popup.js b/src/js/popup.js
index 4f6febe..8f781bf 100644
--- a/src/js/popup.js
+++ b/src/js/popup.js
@@ -28,7 +28,7 @@ import { BaseContainer, BaseList } from './container.js';
 /**
  * @type { import("../typings/types").config }
  */
-let cfg = {};
+let cfg = JSON.parse(JSON.stringify(DEFAULT_CONFIG));
 /******************************************************************************/
 
 /**
@@ -1138,7 +1138,9 @@ ael(main, 'click', async (evt) => {
       openInTab(dataset.webpage);
     } else if (cmd === 'fullscreen') {
       const tab = container.Tabs.getActive();
-      openInTab(`${location.href}?host=${tab.dataset.host}`);
+      if (tab) {
+        openInTab(`${location.href}?host=${tab.dataset.host}`);
+      }
     } else if (cmd === 'navigation') {
       for (const e of qsA('mujs-btn', target.parentElement)) {
         if (dom.cl.has(e, 'nav')) continue;
@@ -2074,14 +2076,15 @@ async function init() {
   Object.assign(currentTab, tab);
   let url;
   try {
-    url = new URL(currentTab.url);
+    url = new URL(currentTab.url || BLANK_PAGE);
     tabURL.href = url.href || '';
   } catch (ex) {
     err(ex);
+    url = new URL(BLANK_PAGE);
   }
 
   if (url !== undefined) {
-    const hostname = formatURL(normalizedHostname(tabURL.hostname));
+    const hostname = url.href === BLANK_PAGE ? BLANK_PAGE : formatURL(normalizedHostname(tabURL.hostname));
     const response = await sendMessage({
       init: true,
       type: 'getData',