I think its obvious where firefox is behind now.
I tried this url.
FishIE Tank
I tested in IE8, IE9, FF4 and FF3.6
The FF3.6 had tons of tabs open, IE9 had about 10 open. FF4 none.
IE9 was the fastest and on weaker spec spare machine, FF4 on same machine bit slower about 25% slower FPS. IE9 unaffected if downloading in background etc. if do things like adjust options on FF4 it slows down.
IE8 page wouldnt load is blank.
FF3.6 aweful, tested on this machine is core i5 750, very low FPS up and down not stable FPS. With just 1 fish it could get 60FPS but struggled whenever I even moved the mouse around the gui without clicking on anything.
So whats clear is that firefox needs to sort out their process isolation stuff, IE loads multiple processes for multiple tabs and it gives it 2 clear advantages. Crash protection and better performance as on a multi core machine a bad tab or GUI issue wont affect another tab as different processes using different cpu cores.
Also firefox has no sandbox which to me is baffling, as the weak point on a typical desktop machine is the browser. I will confess any viruses that have came closest to infecting me have all done so via firefox and I think IE is the more secure browser, what saves firefox is probably the fact IE has a higher userbase so is targeted much more. I still use firefox heavily and will say this is mianly due to tabmixplus excellent addons. Now IE9 supports the lists, firefox still holds a far superior tab session management system and superior password system.
update, after restarting FF4, the fish are smoothish as long as I keep moving the mouse pointer but when I stop moving or have another window in foreground they go super jerky, in this jerky mode it still uses the same cpu usage. So to me IE9 is way ahead on performance efficiency.