diff --git a/PROTOCOL.md b/PROTOCOL.md
index 2512564..810388b 100644
--- a/PROTOCOL.md
+++ b/PROTOCOL.md
@@ -27,7 +27,8 @@ GET /ws/<session>?as=<role>[&injector=<name>]   (WebSocket upgrade)
 A session has at most one host and one observer; a newer connection replaces
 the older one, which is closed with code `4001`. Any number of remotes can
 join. `injector` is free text describing how the host got injected
-(`bookmark`, `extension_chrome`) and is shown in the connection info.
+(`bookmark`, `extension_chrome`, `extension_firefox`) and is shown in the
+connection info.
 
 The first frame the server sends is always `init`:
 
diff --git a/static/js/indexController.js b/static/js/indexController.js
index 35f3b39..b4cb441 100644
--- a/static/js/indexController.js
+++ b/static/js/indexController.js
@@ -26,17 +26,18 @@ slideApp.controller("indexController", ["$scope", "$http", "$cookies", "$timeout
                 console.log(data.info)
                 $scope.statusIcon.showMessage("check", "lime", [200], "Connected", 2000);
 
-                // Notify extension
-                try {
-                    chrome.runtime.sendMessage($scope.extension.id.chrome, {session: $scope.session}, function (msg) {
-                    });
+                // Notify extension. If it opened this tab just to get a session, close
+                // the tab once the extension has it - or after a second at the latest,
+                // as extension versions up to 1.2.0 never answer this message.
+                var closeSessionOnlyTab = function () {
                     if ($state.params.sessionOnly) {
                         $timeout(function () {
                             window.close();
                         }, 100);
                     }
-                } catch (ignoerd) {
-                }
+                };
+                $scope.extension.send({session: $scope.session}, closeSessionOnlyTab);
+                $timeout(closeSessionOnlyTab, 900);
             });
         } else if (data.state == "not_found") {// This should be impossible here, since the observer generates the new session ID
             console.warn("Session not found");
diff --git a/static/js/mainController.js b/static/js/mainController.js
index 265307f..41915f2 100644
--- a/static/js/mainController.js
+++ b/static/js/mainController.js
@@ -94,17 +94,41 @@ slideApp.controller("slideController", ["$scope", "$cookies", "$location", "$htt
             // chrome: "enmfbjneielhjagdgebkmcdhgbbionek"
             chrome: "ojmlekgoiolmbfddhkdbnlpndbcipfkp"
         },
-        installed: false
-    };
-    try {
-        chrome.runtime.sendMessage($scope.extension.id.chrome, {ping: "hello"}, function (msg) {
-            console.log(msg)
-            if (msg && msg.pong && msg.pong == 'hello') {
-                $scope.extension.installed = true;
+        installed: false,
+        // Chrome exposes chrome.runtime to this site once the extension is
+        // installed (externally_connectable). Firefox doesn't let pages message an
+        // extension at all, so there the extension's siteBridge.js content script
+        // relays what is posted to this window, and posts the answer back.
+        send: function (message, callback) {
+            if (window.chrome && chrome.runtime && chrome.runtime.sendMessage) {
+                try {
+                    chrome.runtime.sendMessage($scope.extension.id.chrome, message, function (response) {
+                        void chrome.runtime.lastError;
+                        if (callback) callback(response);
+                    });
+                } catch (ignored) {
+                }
+                return;
             }
-        });
-    } catch (ignoerd) {
-    }
+            var id = ++extensionBridge.lastId;
+            if (callback) extensionBridge.callbacks[id] = callback;
+            window.postMessage({channel: "remoteslide", direction: "to-extension", id: id, message: message}, window.location.origin);
+        }
+    };
+    var extensionBridge = {lastId: 0, callbacks: {}};
+    window.addEventListener("message", function (event) {
+        var data = event.data;
+        if (event.source !== window || !data || data.channel !== "remoteslide" || data.direction !== "to-page") return;
+        var callback = extensionBridge.callbacks[data.id];
+        delete extensionBridge.callbacks[data.id];
+        if (callback) callback(data.response);
+    });
+    $scope.extension.send({ping: "hello"}, function (msg) {
+        console.log(msg)
+        if (msg && msg.pong && msg.pong == 'hello') {
+            $scope.extension.installed = true;
+        }
+    });
 
     $scope.isMobile = function () {
         return window.mobilecheck();