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

@chrismore r?
After my review last night, I was thinking about suggesting moving everything to JS, and after @pmclanahan comment on the caching, it seems like the correct path. We can leverage/repurpose the logic in the [Firefox desktop JS](https://github.com/mozilla/bedrock/blob/master/media/js/firefox/desktop/common.js#L13) to hide and show the appropriate download buttons & messaging.