Firefox port

[MarZab]

As this is a webextension there should not be any reason to limit it to Chrome, so I went ahead and did a port and built/signed a firefox version based of e2b9adf

https://github.com/MarZab/firefox-virtual-keyboard/releases

Since the addon uses exec everywhere, I'm not going to publish it to the store - it's going to get rejected anyway.