Fix Bug 1072988, show conditional content based on old Fx or non Fx browsers

@chrismore r?
After thinking about it, is the "Your Firefox Version Status" headline even necessary? Would removing it cause any confusion for a user? Doesn't "Looks like you’re using an older version of Firefox." say it all? We could increase the font size of that already localized message to make it a bit more prominent.

This would save localizers time (which is very short at the moment), and save us from adding an `l10n_has_tag` block.