I am still using firefox 2, and I must say, that with gmail, it is almost intolerably slow.
To click the link for this auto generated email, I have to wait at least 7 seconds before I am able to get the cursor in the edit box and type. Also, with the long threads... sometimes, scrolling these will actually cause such disruption in my computers processing that playing mp3's (rhythmbox) will pause for a few seconds before resuming once firefox has recovered.
As of today, I still haven't tested firefox 3 b5 on anything but ubuntu hardy LIVE, and so it wasn't a fair test run (as it was running from CD). I will give futher progress reports when I finally install firefox3 (I am tossing up whether or not to replace the small 30gig harddrive first).