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

@chrismore r?
The caching issue is only with our frontend cache in production (zeus). It will serve the cached version regardless of browser. I don't think we should change this behavior either as this is one of the highest-traffic pages on the site, and would significantly increase load on the servers. But client-side has no such issue as the same JS is served to everyone and it decides what to do. So it'll be better to use JS to detect browsers (for which we already have decent code) and show and hide elements based on that. We do a similar thing quite a lot on /firefox/new/.