On my Macbook running Mac OS X 10.6.7, FF 4 leaks memory like crazy. I used to let FF 3.6 open for days if not weeks, with 30 to 40 tabs open. Now FF 4 gobbles up to 1 GB of RAM after browsing to 10/20 sites, at which point I must restart it before the system becomes unusably slow.
Once it restarted in 32 bits mode, because of a plugin (perian?) and it was much less memory-hungry in 32 bits than 64; it still isn't usable for long this way.
Can anyone confirm if is a configuration related bug, or a bug with the FF4 core on Mac?
Update 1:
I confirm that FF behaves properly when running in safe mode. Apparently it still behaves normally with AdBlock+, Gmail checker and Tree Style Tabs on. I'm now testing one extension at a time. The most probable suspects so far are GreaseMonkey and Firebug.
Update 2:
Apparently the culprit is the DownloadHelper extension. I'll come back when I've got it tested some more.
Update 3:
It looks like GreaseMonkey leaks somewhat too, but I'll have to test for a couple of days to be sure.
Update 4:
Firebug seems to be leaking, and the Gmail checker too.
Update 5:
I've now opened cases on FF and DownloadHelper bugzillas.