diff --git a/browser-extension/prepareExtension.js b/browser-extension/prepareExtension.js
index 9e3db16c7d..dfcb1820a1 100644
--- a/browser-extension/prepareExtension.js
+++ b/browser-extension/prepareExtension.js
@@ -1,5 +1,5 @@
 import {
-  getForEnv,
+  selectValue,
   generateImportWrappers,
   writeManifest,
   writeConfig,
@@ -8,43 +8,94 @@ import {
 
 const env = process.env.TOURNESOL_ENV || 'production';
 
+const browser = process.env.EXTENSION_BROWSER || 'firefox';
+if (process.env.EXTENSION_BROWSER && !process.env.MANIFEST_VERSION) {
+  throw new Error(`MANIFEST_VERSION is required with EXTENSION_BROWSER`);
+}
+
+const manifestVersion = parseInt(process.env.MANIFEST_VERSION || 2);
+if (manifestVersion != 2 && manifestVersion != 3)
+  throw new Error(`Invalid manifest version: ${manifestVersion}`);
+if (manifestVersion === 2) {
+  console.info(
+    `Extension will be configured with manifest version ${manifestVersion}.`
+  );
+} else {
+  console.info(
+    `Extension will be configured for ${browser} with manifest version ${manifestVersion}.`
+  );
+}
+
 const { version } = await readPackage();
+const hostPermissions = [
+  ...selectValue(env, {
+    production: ['https://tournesol.app/', 'https://api.tournesol.app/'],
+    'dev-env': [
+      'http://localhost/',
+      'http://localhost:3000/',
+      'http://localhost:8000/',
+    ],
+  }),
+  'https://www.youtube.com/',
+];
+
+const permissions = [
+  'activeTab',
+  'contextMenus',
+  'storage',
+  'webNavigation',
+  // webRequest and webReauestBlocking were used to overwrite
+  // headers in the API response. This is no longer the case
+  // with version > 3.5.2.
+  // These permissions can be removed as soon as we are confident
+  // the next release works as expected.
+  'webRequest',
+  'webRequestBlocking',
+  ...selectValue(manifestVersion, { 2: [], 3: ['scripting'] }),
+];
+
+const allPermissions = selectValue(manifestVersion, {
+  2: { permissions: [...hostPermissions, ...permissions] },
+  3: { permissions, host_permissions: hostPermissions },
+});
+
+const webAccessibleResourcesFromYouTube = [
+  'Logo128.png',
+  'html/*',
+  'images/*',
+  'utils.js',
+  'models/*',
+  'config.js',
+];
 
 const manifest = {
   name: 'Tournesol Extension',
   version,
   description: 'Open Tournesol directly from YouTube',
-  permissions: [
-    ...getForEnv(
-      {
-        production: ['https://tournesol.app/', 'https://api.tournesol.app/'],
-        'dev-env': [
-          'http://localhost/',
-          'http://localhost:3000/',
-          'http://localhost:8000/',
-        ],
-      },
-      env
-    ),
-    'https://www.youtube.com/',
-    'activeTab',
-    'contextMenus',
-    'storage',
-    'webNavigation',
-    'webRequest',
-    'webRequestBlocking',
-  ],
-  manifest_version: 2,
+  ...allPermissions,
+  manifest_version: manifestVersion,
   icons: {
     64: 'Logo64.png',
     128: 'Logo128.png',
     512: 'Logo512.png',
   },
-  background: {
-    page: 'background.html',
-    persistent: true,
-  },
-  browser_action: {
+  background: selectValue(manifestVersion, {
+    2: { page: 'background.html', persistent: true },
+    3: selectValue(browser, {
+      // It's possible to make a browser-independent background value but
+      // Chrome only supports that since version 121 released in January 2024.
+      // See https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/background
+      firefox: {
+        scripts: ['background.js'],
+        type: 'module',
+      },
+      chrome: {
+        service_worker: 'background.js',
+        type: 'module',
+      },
+    }),
+  }),
+  [selectValue(manifestVersion, { 2: 'browser_action', 3: 'action' })]: {
     default_icon: {
       16: 'Logo16.png',
       64: 'Logo64.png',
@@ -68,13 +119,10 @@ const manifest = {
       all_frames: true,
     },
     {
-      matches: getForEnv(
-        {
-          production: ['https://tournesol.app/*'],
-          'dev-env': ['http://localhost:3000/*'],
-        },
-        env
-      ),
+      matches: selectValue(env, {
+        production: ['https://tournesol.app/*'],
+        'dev-env': ['http://localhost:3000/*'],
+      }),
       js: [
         'fetchTournesolToken.js',
         'fetchTournesolRecommendationsLanguages.js',
@@ -88,20 +136,28 @@ const manifest = {
     open_in_tab: true,
   },
   default_locale: 'en',
-  web_accessible_resources: [
-    'Logo128.png',
-    'html/*',
-    'images/*',
-    'utils.js',
-    'models/*',
-    'config.js',
-  ],
+  web_accessible_resources: selectValue(manifestVersion, {
+    2: webAccessibleResourcesFromYouTube,
+    3: [
+      {
+        matches: [
+          'https://*.youtube.com/*',
+          selectValue(env, {
+            production: 'https://tournesol.app/*',
+            'dev-env': 'http://localhost:3000/*',
+          }),
+        ],
+        resources: webAccessibleResourcesFromYouTube,
+      },
+    ],
+  }),
 };
 
 // Please DO NOT add a trailing slash to front end URL, this prevents
 // creating duplicates in our web analytics tool
-const config = getForEnv(
-  {
+const config = {
+  manifestVersion,
+  ...selectValue(env, {
     production: {
       frontendUrl: 'https://tournesol.app',
       frontendHost: 'tournesol.app',
@@ -112,12 +168,11 @@ const config = getForEnv(
       frontendHost: 'localhost:3000',
       apiUrl: 'http://localhost:8000',
     },
-  },
-  env
-);
+  }),
+};
 
 (async () => {
-  await generateImportWrappers(manifest);
+  await generateImportWrappers(manifest, webAccessibleResourcesFromYouTube);
   await writeManifest(manifest, 'src/manifest.json');
   await writeConfig(config, 'src/config.js');
 })();
diff --git a/browser-extension/prepareTools.js b/browser-extension/prepareTools.js
index 37cc214ec6..a7479fe55b 100644
--- a/browser-extension/prepareTools.js
+++ b/browser-extension/prepareTools.js
@@ -1,17 +1,18 @@
 import { writeFile, mkdir, readFile } from 'node:fs/promises';
 import { dirname, join } from 'node:path';
 
-export const getForEnv = (object, env) => {
-  const result = object[env];
+export const selectValue = (key, options) => {
+  const result = options[key];
   if (result === undefined) {
-    throw new Error(
-      `No value found for the environment ${JSON.stringify(env)}`
-    );
+    throw new Error(`No value found for the key ${JSON.stringify(key)}`);
   }
   return result;
 };
 
-export const generateImportWrappers = async (manifest) => {
+export const generateImportWrappers = async (
+  manifest,
+  webAccessibleResources
+) => {
   await Promise.all(
     manifest['content_scripts'].map(async (contentScript) => {
       await Promise.all(
@@ -22,7 +23,7 @@ export const generateImportWrappers = async (manifest) => {
           await mkdir(dirname(path), { recursive: true });
           await writeFile(path, content);
           contentScript.js[i] = newJs;
-          manifest['web_accessible_resources'].push(js);
+          webAccessibleResources.push(js);
         })
       );
     })
diff --git a/browser-extension/src/.eslintrc.json b/browser-extension/src/.eslintrc.json
index 2659a2c24a..24c9f91303 100644
--- a/browser-extension/src/.eslintrc.json
+++ b/browser-extension/src/.eslintrc.json
@@ -6,7 +6,7 @@
     "webextensions": true
   },
   "parserOptions": {
-    "ecmaVersion": 11,
+    "ecmaVersion": 12,
     "sourceType": "module",
     "ecmaFeatures": {
       "modules": true
diff --git a/browser-extension/src/background.js b/browser-extension/src/background.js
index 6a81596e29..dac140e174 100644
--- a/browser-extension/src/background.js
+++ b/browser-extension/src/background.js
@@ -31,10 +31,10 @@ const createContextMenu = function createContextMenu() {
     });
   });
 
-  chrome.contextMenus.onClicked.addListener(function (e) {
+  chrome.contextMenus.onClicked.addListener(function (e, tab) {
     var videoId = new URL(e.linkUrl).searchParams.get('v');
     if (!videoId) {
-      alertUseOnLinkToYoutube();
+      alertUseOnLinkToYoutube(tab);
     } else {
       addRateLater(videoId).then((response) => {
         if (!response.success) {
@@ -47,7 +47,8 @@ const createContextMenu = function createContextMenu() {
                 function (response) {
                   if (!response.success) {
                     alertOnCurrentTab(
-                      'Sorry, an error occured while opening the Tournesol login form.'
+                      'Sorry, an error occured while opening the Tournesol login form.',
+                      tab
                     );
                   }
                 }
@@ -61,32 +62,6 @@ const createContextMenu = function createContextMenu() {
 };
 createContextMenu();
 
-/**
- * Remove the X-FRAME-OPTIONS and FRAME-OPTIONS headers included in the
- * Tournesol application HTTP answers. It allows the extension to display
- * the application in an iframe without enabling all website to do the same.
- */
-chrome.webRequest.onHeadersReceived.addListener(
-  function (info) {
-    const headers = info.responseHeaders.filter(
-      (h) =>
-        !['x-frame-options', 'frame-options'].includes(h.name.toLowerCase())
-    );
-    return { responseHeaders: headers };
-  },
-  {
-    urls: ['https://tournesol.app/*'],
-    types: ['sub_frame'],
-  },
-  [
-    'blocking',
-    'responseHeaders',
-    // Modern Chrome needs 'extraHeaders' to see and change this header,
-    // so the following code evaluates to 'extraHeaders' only in modern Chrome.
-    chrome.webRequest.OnHeadersReceivedOptions.EXTRA_HEADERS,
-  ].filter(Boolean)
-);
-
 function getDateThreeWeeksAgo() {
   // format a string to properly display years months and day: 2011 -> 11, 5 -> 05, 12 -> 12
   const threeWeeksAgo = new Date(Date.now() - 3 * 7 * 24 * 3600000);
diff --git a/browser-extension/src/utils.js b/browser-extension/src/utils.js
index 94056d0a77..e56a73d0b1 100644
--- a/browser-extension/src/utils.js
+++ b/browser-extension/src/utils.js
@@ -1,4 +1,4 @@
-import { apiUrl } from './config.js';
+import { apiUrl, manifestVersion } from './config.js';
 
 export const getAccessToken = async () => {
   return new Promise((resolve) => {
@@ -8,14 +8,32 @@ export const getAccessToken = async () => {
   });
 };
 
-export const alertOnCurrentTab = async (msg) => {
-  chrome.tabs.executeScript({
-    code: `alert("${msg}", 'ok')`,
-  });
+const getCurrentTab = async () => {
+  const queryOptions = { active: true, lastFocusedWindow: true };
+  const [tab] = await chrome.tabs.query(queryOptions);
+  return tab;
+};
+
+export const alertOnCurrentTab = async (msg, tab) => {
+  if (manifestVersion === 2) {
+    chrome.tabs.executeScript({
+      code: `alert("${msg}", 'ok')`,
+    });
+  } else {
+    tab ??= await getCurrentTab();
+    const windowAlert = (msg, btn) => {
+      window.alert(msg, btn);
+    };
+    chrome.scripting.executeScript({
+      target: { tabId: tab.id },
+      func: windowAlert,
+      args: [msg, 'ok'],
+    });
+  }
 };
 
-export const alertUseOnLinkToYoutube = () => {
-  alertOnCurrentTab('This must be used on a link to a youtube video');
+export const alertUseOnLinkToYoutube = (tab) => {
+  alertOnCurrentTab('This must be used on a link to a youtube video', tab);
 };
 
 export const fetchTournesolApi = async (path, options = {}) => {