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

@chrismore r?
I think testing for `Firefox` alone can give false positives in Seamonkey & Iceweasel (and Camino, if anyone is still running that). We've got a slightly more robust JS test for this in [global.js](https://github.com/mozilla/bedrock/blob/master/media/js/base/global.js#L86). I could see us needing/using this functionality on the Django side in the future. Maybe worth creating a helper in [firefox/utils.py](https://github.com/mozilla/bedrock/blob/master/bedrock/firefox/utils.py)?

Wouldn't mind hearing from @pmclanahan and/or @jgmize on this.