Add compatibility with Firefox for desktop

package.json

@@ -23,7 +23,8 @@
     "private-browsing": true
   },
   "engines": {
-    "fennec": ">=45.0a1 <=*"
+    "fennec": ">=45.0a1 <=*",
+    "firefox": ">=45.0a1 <=*"
   },
   "keywords": [
     "browser",