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

@chrismore r?
Ooooooh, I see what you mean. Just tested against prod using a local install of Fx30 and then Aurora in a VM and Aurora was definitely getting the cached version. On refresh it fixes it but, not ideal.

So that means that neither the is_latest nor is_firefox bits will work reliably. I am guessing this all needs to move to JS then, or will there be a caching issue as well?