Improve backward compatibility

lib/main.js

@@ -17,8 +17,8 @@
  * Imports
  */
 
-var webextension = require('sdk/webextension');
 var preferences = require('sdk/simple-prefs');
+var webextension = null;
 
 var self = require('sdk/self');
 var tabs = require("sdk/tabs");
@@ -66,6 +66,12 @@ exports.main = function (options) {
         }
     }
 
+    try {
+        webextension = require('sdk/webextension');
+    } catch (exception) {
+        return;
+    }
+
     // Initialize the embedded WebExtension.
     webextension.startup().then(({ browser }) => {
 
@@ -113,7 +119,7 @@ exports.onUnload = function () {
 // Sends injection updates to the WebExtension.
 exports.broadcastInjection = function () {
 
-    if (typeof webextensionPort === 'object') {
+    if (webextensionPort !== null) {
 
         webextensionPort.postMessage({
             'subject': 'register-injection'

package.json

@@ -13,8 +13,8 @@
     "multiprocess": true
   },
   "engines": {
-    "firefox": ">=51.0a1",
-    "fennec": ">=51.0a1",
+    "firefox": ">=38.0a1",
+    "fennec": ">=38.0a1",
     "seamonkey": ">=2.0a1 <=2.46",
     "{8de7fcbb-c55c-4fbe-bfc5-fc555c87dbc4}": ">=27.1.0b1"
   }