Update the manifest code.

source/manifest.ts

@@ -1,11 +1,17 @@
 /* eslint-disable @typescript-eslint/naming-convention */
 
-export default function createManifest(
-  target: string,
-): Record<string, unknown> {
-  const manifest: Record<string, unknown> = {
+import {type Manifest} from 'webextension-polyfill';
+
+/**
+ * Creates the WebExtension manifest based on the browser target.
+ *
+ * @param browser The browser target ("firefox" or "chromium").
+ * @returns The WebExtension manifest.
+ */
+export function createManifest(browser: string): Manifest.WebExtensionManifest {
+  const manifest: Manifest.WebExtensionManifest = {
+    manifest_version: Number.NaN,
     name: 'Queue',
-    description: 'A WebExtension for queueing links.',
     version: '0.3.2',
     permissions: ['contextMenus', 'storage'],
     options_ui: {
@@ -14,36 +20,41 @@ export default function createManifest(
     },
   };
 
-  const icons = {
+  const icons: Manifest.IconPath = {
     128: 'queue.png',
   };
 
-  manifest.icons = icons;
-
-  const browserAction = {
+  const action: Manifest.ActionManifest = {
     default_icon: icons,
   };
 
-  const backgroundScript = 'background-scripts/initialize.ts';
+  const backgroundScript = 'background/setup.js';
 
-  if (target === 'chromium') {
+  if (browser === 'firefox') {
+    manifest.manifest_version = 2;
+    manifest.background = {
+      scripts: [backgroundScript],
+    };
+    manifest.browser_action = action;
+    manifest.browser_specific_settings = {
+      gecko: {
+        id: '{c3560e6b-00e5-4ab3-b89e-8a54ee5b2c9f}',
+        strict_min_version: '102.0',
+      },
+    };
+  } else if (browser === 'chromium') {
     manifest.manifest_version = 3;
-    manifest.action = browserAction;
+    manifest.action = action;
     manifest.background = {
       service_worker: backgroundScript,
       type: 'module',
     };
   } else {
-    manifest.manifest_version = 2;
-    manifest.browser_action = browserAction;
-    manifest.background = {
-      scripts: [backgroundScript],
-    };
-    manifest.applications = {
-      gecko: {
-        id: '{c3560e6b-00e5-4ab3-b89e-8a54ee5b2c9f}',
-      },
-    };
+    throw new Error(`Unknown target browser: ${browser}`);
+  }
+
+  if (Number.isNaN(manifest.manifest_version)) {
+    throw new TypeError('Manifest version is NaN');
   }
 
   return manifest;