AFAIK Firefox OS is still using Firefox 18, which is old enough to still have a bug which can kill performance. Hopefully a future update will bring them up to Firefox 25+ which should be nice and fast.
The stock browser on Android is pretty poor - try Chrome on the Xperia Ray if you weren't using it already and it should be a lot faster.
And the Nexus 7 seems to be working nicely :)