[fix bug 1006617] Firefox Facebook app shows the old logo and screenshots

* Updates the Facebook download app with new assets (borrowed from `firefox/new`).
* This PR also goes part way to addressing [bug 911312](https://bugzilla.mozilla.org/show_bug.cgi?id=911312), by showing the correct Linux browser image. For now I have hidden the download instructions for Linux users since we don't have any appropriate copy or images (this is also what we do on `/firefox/new`).