Firefox 6 got pushed on my Ubuntu 11.04 netbook as an update a couple of days ago and things were badly broken; memory usage was skyrocketing, the kswapd was eating almost all CPU and the system was pretty much in a continuous wait-state.
After some cursing and a lot of Scroogling, I finally stumbled across this blogpost which described the exact same problem and advised to set “layers.acceleration.force-enabled” (which tries to force hardware acceleration, which isn’t supported on Linux by default) back to “false” in about:config. And indeed this small rollback solved my memory-woes. Guess I shouldn’t have dismissed that silly about:config warning message after all.
And while we’re on the subject; Firefox 7 should see substantial improvements in memory usage, yay!