Comment 35 for bug 144216

Same issue in Hardy running on a Dell M4300, Intel 9500, 4GB ram, nvidia 360M 256mb (1900x1200), compiz, bleeding-edge nv drivers. As a rough guess, I'd say that switching tabs under FF3 is about 1/10th the speed it should be.
I thought it may be worth adding that canvas (e.g. www.blobsallad.se) is also incredibly slow on FF3 but at normal speed under FF2. On previous computers slow canvas was an indicator of a gfx driver issue. There is a time difference between switching tabs on FF3 v. FF2, but it's slight.

https://bugs.launchpad.net/ubuntu/+source/xulrunner-1.9/+bug/185469 Looks a lot like this issue.