Switch firefox from kiosk mode to fullscreen mode

[jreidinger]

The change add extension for starting in screen mode as now there is no simple way how to start ff in fullscreen mode. Extension is simple 15 lines and was manually reviewed by me. License is MIT.

[lslezak]

Please update the live/README.md documentation why we install the plugin. Please add also some links so we know where it was downloaded from and could possibly update it later.

[jreidinger]

@lslezak updated with requested info

[dgdavid]

@jreidinger, the work here might be closing #341

[jreidinger]

@dgdavid I see more issues that can be closed with it, but lets see how wayland will go as this kind of expect it to be done first.