diff --git a/background.js b/background.js
index 323105f..6ee71d1 100644
--- a/background.js
+++ b/background.js
@@ -1,9 +1,32 @@
-importScripts("sessionGrabber.js");
+// Chrome runs this file as a service worker, which pulls in its siblings itself.
+// Firefox has no background service workers and runs it as an event page
+// instead, with sessionGrabber.js already loaded from background.scripts.
+if (typeof importScripts === "function") {
+    importScripts("sessionGrabber.js");
+}
 
-chrome.runtime.onMessageExternal.addListener(function (msg, sender, sendResponse) {
-    if (msg.ping && msg.ping == 'hello') {
-        sendResponse({pong: "hello"});
+// Messages from remote-sli.de. Chrome lets the site send them here directly
+// (externally_connectable); Firefox doesn't let web pages message an extension
+// at all, so there util/siteBridge.js relays what the site posts to its window.
+function handleSiteMessage(msg, sendResponse) {
+    if (msg.session) {
+        grabSession(msg.session);
+    }
+    // Always answer: the site closes the tab it opened for a session once it hears back
+    sendResponse(msg.ping == 'hello' ? {pong: "hello"} : {});
+}
+
+function isSiteSender(sender) {
+    try {
+        var host = new URL(sender.url).hostname;
+        return host == "remote-sli.de" || host.endsWith(".remote-sli.de");
+    } catch (ignored) {
+        return false;
     }
+}
+
+chrome.runtime.onMessageExternal.addListener(function (msg, sender, sendResponse) {
+    handleSiteMessage(msg, sendResponse);
 });
 
 // The badge belongs to the tab the controller runs in. Messages from the injected
@@ -27,6 +50,9 @@ function setRemoteCountBadge(sender, data) {
 
 // Handle messages from the pageController script
 chrome.runtime.onMessage.addListener(function (msg, sender, sendResponse) {
+    if (msg.action == 'siteMessage' && isSiteSender(sender)) {
+        handleSiteMessage(msg.message || {}, sendResponse);
+    }
     if (msg.action == 'socketEvent') {
         var event = msg.event;
         var data = msg.data;
diff --git a/index.js b/index.js
index abafb37..36b5f10 100644
--- a/index.js
+++ b/index.js
@@ -1,5 +1,8 @@
 var app = angular.module("app", []);
 
+// Shown in the session's connection info, so tell the two builds apart
+var INJECTOR = chrome.runtime.getURL("").indexOf("moz-extension:") === 0 ? "extension_firefox" : "extension_chrome";
+
 // chrome.tabs.getSelected is gone in MV3; the popup always belongs to the current window.
 function withActiveTab(callback) {
     chrome.tabs.query({active: true, currentWindow: true}, function (tabs) {
@@ -71,7 +74,7 @@ app.controller("mainCtrl", function ($scope, $timeout) {
                         },
                         args: [{
                             session: $scope.session.session,
-                            injector: 'extension_chrome'
+                            injector: INJECTOR
                         }]
                     });
                 }).then(function () {
diff --git a/inject/controller b/inject/controller
index b9fe1f8..7f11f75 160000
--- a/inject/controller
+++ b/inject/controller
@@ -1 +1 @@
-Subproject commit b9fe1f8002b5cc7a11140b4ec5947cf991c139ae
+Subproject commit 7f11f75ce07e102795953f0c9c89fac13f0ee7e6
diff --git a/manifest.json b/manifest.json
index 433750a..01f5508 100644
--- a/manifest.json
+++ b/manifest.json
@@ -5,7 +5,18 @@
   "version": "1.2.0",
   "author": "inventivetalent",
   "homepage_url": "https://remote-sli.de",
-  "minimum_chrome_version": "102",
+  "minimum_chrome_version": "121",
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "remoteslide@inventivetalent.org",
+      "strict_min_version": "140.0",
+      "data_collection_permissions": {
+        "required": [
+          "websiteContent"
+        ]
+      }
+    }
+  },
   "action": {
     "default_icon": "img/logo-128.png",
     "default_popup": "index.html"
@@ -32,7 +43,11 @@
     "extension_pages": "script-src 'self'; object-src 'self'"
   },
   "background": {
-    "service_worker": "background.js"
+    "service_worker": "background.js",
+    "scripts": [
+      "sessionGrabber.js",
+      "background.js"
+    ]
   },
   "content_scripts": [
     {
@@ -54,6 +69,15 @@
         "util/preziInjector.js"
       ],
       "all_frames": true
+    },
+    {
+      "matches": [
+        "*://*.remote-sli.de/*"
+      ],
+      "js": [
+        "util/siteBridge.js"
+      ],
+      "run_at": "document_start"
     }
   ],
   "web_accessible_resources": [
diff --git a/sessionGrabber.js b/sessionGrabber.js
index 4027353..a48727d 100644
--- a/sessionGrabber.js
+++ b/sessionGrabber.js
@@ -1,24 +1,22 @@
-console.log("Waiting for session message...")
-chrome.runtime.onMessageExternal.addListener(function (msg, sender, sendResponse) {
-    if (msg.session) {
-        console.log("Session msg: " + JSON.stringify(msg))
-        chrome.storage.local.get(["session"], function (items) {
-            var oldSession = items.session;
+// Stores a session that remote-sli.de handed over (see handleSiteMessage in background.js)
+function grabSession(session) {
+    console.log("Session msg: " + JSON.stringify(session))
+    chrome.storage.local.get(["session"], function (items) {
+        var oldSession = items.session;
 
-            msg.session.sessionTime = new Date().valueOf();
-            chrome.storage.local.set({"session": msg.session});
-            if (!oldSession || oldSession.session != msg.session.session) {
-                console.log("notification created")
-                chrome.notifications.create('new-session-created-notification', {
-                    type: 'basic',
-                    iconUrl: 'img/logo-128.png',
-                    title: 'Session Updated',
-                    message: 'A new session has been created\n' +
-                    'Click the icon to start presenting!',
-                    contextMessage: msg.session.session
-                }, function (notificationId) {
-                });
-            }
-        });
-    }
-});
\ No newline at end of file
+        session.sessionTime = new Date().valueOf();
+        chrome.storage.local.set({"session": session});
+        if (!oldSession || oldSession.session != session.session) {
+            console.log("notification created")
+            chrome.notifications.create('new-session-created-notification', {
+                type: 'basic',
+                iconUrl: 'img/logo-128.png',
+                title: 'Session Updated',
+                message: 'A new session has been created\n' +
+                'Click the icon to start presenting!',
+                contextMessage: session.session
+            }, function (notificationId) {
+            });
+        }
+    });
+}
diff --git a/util/siteBridge.js b/util/siteBridge.js
new file mode 100644
index 0000000..7cc0280
--- /dev/null
+++ b/util/siteBridge.js
@@ -0,0 +1,20 @@
+// Firefox has no externally_connectable, so remote-sli.de can't message the
+// extension the way it does in Chrome. Instead it posts the same messages to its
+// own window, and this relays them to the background and posts the answers back:
+//   page -> {channel: "remoteslide", direction: "to-extension", id, message}
+//   page <- {channel: "remoteslide", direction: "to-page", id, response}
+window.addEventListener("message", function (event) {
+    var data = event.data;
+    if (event.source !== window || !data || data.channel !== "remoteslide" || data.direction !== "to-extension") {
+        return;
+    }
+    chrome.runtime.sendMessage({action: "siteMessage", message: data.message}, function (response) {
+        void chrome.runtime.lastError;
+        window.postMessage({
+            channel: "remoteslide",
+            direction: "to-page",
+            id: data.id,
+            response: response
+        }, window.location.origin);
+    });
+});