diff --git a/source/background.js b/source/background.js
index 7f70157..382a976 100644
--- a/source/background.js
+++ b/source/background.js
@@ -2,7 +2,9 @@ import addDomainPermissionToggle from 'webext-permission-toggle';
 import oneEvent from 'one-event';
 import optionsStorage from './options-storage.js';
 
-const browserAction = chrome.action ?? chrome.browserAction;
+const browser = globalThis.browser ?? globalThis.chrome;
+
+const browserAction = browser.action ?? browser.browserAction;
 
 // Firefox hates iframes on activeTab
 // https://bugzilla.mozilla.org/show_bug.cgi?id=1653408
@@ -17,17 +19,22 @@ if (navigator.userAgent.includes('Firefox/')) {
 }
 
 function stopGT(tab) {
-	chrome.scripting.executeScript({
+	browser.scripting.executeScript({
 		target: {tabId: tab.id},
 		func: () => stopGT(),
 	});
 }
 
 async function handleAction({id}) {
-	const defaults = {
+	const defaultTarget = {
 		target: {tabId: id, allFrames: true},
 	};
-	const [topFrame] = await chrome.scripting.executeScript({
+	const defaults = {
+		...defaultTarget,
+		injectImmediately: true,
+	};
+
+	const [topFrame] = await browser.scripting.executeScript({
 		...defaults,
 		func: () => typeof window.startGT === 'function',
 	});
@@ -37,15 +44,15 @@ async function handleAction({id}) {
 	if (!alreadyInjected) {
 		try {
 			await Promise.all([
-				chrome.scripting.insertCSS({...defaults, files: ['/ghost-text.css']}),
-				chrome.scripting.executeScript({...defaults, files: ['/ghost-text.js']}),
+				browser.scripting.insertCSS({...defaultTarget, files: ['/ghost-text.css']}),
+				browser.scripting.executeScript({...defaults, files: ['/ghost-text.js']}),
 			]);
 		} catch (error) {
 			console.error(error);
 		}
 	}
 
-	await chrome.scripting.executeScript({...defaults, func: () => startGT()});
+	await browser.scripting.executeScript({...defaults, func: () => startGT()});
 }
 
 function handlePortListenerErrors(listener) {
@@ -69,7 +76,7 @@ function handlePortListenerErrors(listener) {
 	};
 }
 
-chrome.runtime.onConnect.addListener(handlePortListenerErrors(async port => {
+browser.runtime.onConnect.addListener(handlePortListenerErrors(async port => {
 	console.assert(port.name === 'new-field');
 	const options = await optionsStorage.getAll();
 	const response = await fetch(`http://localhost:${options.serverPort}`);
@@ -115,50 +122,50 @@ function handleMessages({code, count}, {tab}) {
 			tabId: tab.id,
 		});
 	} else if (code === 'focus-tab') {
-		chrome.tabs.update(tab.id, {active: true});
-		chrome.windows.update(tab.windowId, {focused: true});
+		browser.tabs.update(tab.id, {active: true});
+		browser.windows.update(tab.windowId, {focused: true});
 	}
 }
 
 // Temporary code from https://github.com/fregante/GhostText/pull/267
 async function saveShortcut() {
-	const storage = await chrome.storage.local.get('shortcut');
+	const storage = await browser.storage.local.get('shortcut');
 	if (storage.shortcut) {
 		// Already saved
 		return;
 	}
 
-	const shortcuts = await chrome.commands.getAll();
+	const shortcuts = await browser.commands.getAll();
 	for (const item of shortcuts) {
 		if (item.shortcut) {
 			// eslint-disable-next-line no-await-in-loop -- Intentional
-			await chrome.storage.local.set({shortcut: item.shortcut});
+			await browser.storage.local.set({shortcut: item.shortcut});
 			return;
 		}
 	}
 }
 
 async function getActiveTab() {
-	const [activeTab] = await chrome.tabs.query({active: true, currentWindow: true});
+	const [activeTab] = await browser.tabs.query({active: true, currentWindow: true});
 	return activeTab;
 }
 
 function init() {
 	browserAction.onClicked.addListener(handleAction);
-	chrome.runtime.onMessage.addListener(handleMessages);
-	chrome.contextMenus.create({
+	browser.runtime.onMessage.addListener(handleMessages);
+	browser.contextMenus.create({
 		id: 'stop-gt',
 		title: 'Disconnect GhostText on this page',
 		contexts: ['action'],
 	});
-	chrome.contextMenus.onClicked.addListener(({menuItemId}, tab) => {
+	browser.contextMenus.onClicked.addListener(({menuItemId}, tab) => {
 		if (menuItemId === 'stop-gt') {
 			stopGT(tab);
 		} else if (menuItemId === 'start-gt-editable') {
 			handleAction(tab);
 		}
 	});
-	chrome.commands.onCommand.addListener(async (command, tab = getActiveTab()) => {
+	browser.commands.onCommand.addListener(async (command, tab = getActiveTab()) => {
 		if (command === 'open') {
 			handleAction(await tab);
 		} else if (command === 'close') {
@@ -166,7 +173,7 @@ function init() {
 		}
 	});
 
-	chrome.contextMenus.create({
+	browser.contextMenus.create({
 		id: 'start-gt-editable',
 		title: 'Activate GhostText on field',
 		contexts: ['editable'],
@@ -176,15 +183,15 @@ function init() {
 		color: '#008040',
 	});
 
-	chrome.runtime.onInstalled.addListener(async ({reason}) => {
+	browser.runtime.onInstalled.addListener(async ({reason}) => {
 		// Only notify on install
 		if (reason === 'install') {
-			const {installType} = await chrome.management.getSelf();
+			const {installType} = await browser.management.getSelf();
 			if (installType === 'development') {
 				return;
 			}
 
-			await chrome.tabs.create({
+			await browser.tabs.create({
 				url: 'https://ghosttext.fregante.com/welcome/',
 				active: true,
 			});
diff --git a/source/manifest.json b/source/manifest.json
index aca33c7..ac8079e 100644
--- a/source/manifest.json
+++ b/source/manifest.json
@@ -14,6 +14,9 @@
 		"128": "icons/icon128.png",
 		"256": "icons/icon256.png"
 	},
+	"content_security_policy": {
+		"extension_pages": "connect-src 'self' http://localhost:* ws:; script-src 'self';"
+	},
 	"permissions": [
 		"activeTab",
 		"contextMenus",