I think "firefox is slow" partialy comes from people using css3 animations eg. this
http://bartaz.github.io/impress.js/#/bored
works smooth on my MBP as well as on my Ubuntu desktop
whereas seems sluggish on FF (on both platforms)
Looks pretty smooth to me in the latest Firefox (Debian testing), but if somewhere it's a bottleneck, to improve that Firefox should fully implement OMTC.