Improved compatibility with slightly older versions of Firefox.

However before Firefox 22 you still have to manually enable flexbox. But now
there is a warning, telling the user to do so.

Users with older versions than Firefox 18, get a warning to update their
browser instead, because of flexbox isn't available at all.
Fixed behaviour in Fennec.

The 'keyup' event was replaced by the 'input' event, because of due to a bug in
Fennic 'keyup' is never fired. However it looks like that the 'input' event is
rather what we want here, anyway.

Browser autocompletion was disabled. For some reason it was only active in
Fennic. And resulted in overlaying other input elements with the suggestions.