Unfortunately the possible ways of interacting with the browser's user interface on Firefox for Android are currently alot more restricted than on desktop Firefox (the fact that I can add an icon to the browser's address bar is already a hack in itself!). As such I had decided to halt all further Firefox for Android development until this is being addressed.

As of Firefox for Android 55 this is finally the case and the first set of improvements (no configurable list yet, unfortunately) has landed in the recently reviewed 1.0.5 version of the extension.

If there are user-agents that you would like to see in the next release in particular (somebody already requested iPad), then please add that to your comment and I'll probably add them. The user-expandable list should be released in not-too-long as well.

Cheers!