I don't have any experience with very low powered Windows phones. I'd be very surprised if the OS / basic apps performed as well as the FF OS ones. But there would be a better range of apps on windows phone (for now).
I guess it depends what the priorities are.
Even the low end Windows Phone 7 devices perform a lot better. I get what people are saying about the WP8 devices, but things like the W1 are double the price. WP7 is still very relevant as there is still a large install base that developers won't ignore. Firefox on the other hand has hardly any apps and I can't see that changing that soon (he says while preparing a second Firefox app submission
