I believe one of the key reasons for dropping XUL extensions is because they don't work very well with e10s (multiprocess Firefox). I recently had occasion to u

I believe one of the key reasons for dropping XUL extensions is because they don't work very well with e10s (multiprocess Firefox). I recently had occasion to use a fresh Firefox without any e10s-incompatible extensions installed, and it really felt much faster, so I think Firefox-future may also qualify as "fast browser".