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?
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.
Apparently the culprit is the DownloadHelper extension. I'll come back when I've got it tested some more.
It looks like GreaseMonkey leaks somewhat too, but I'll have to test for a couple of days to be sure.
Firebug seems to be leaking, and the Gmail checker too.
I've now opened cases on FF and DownloadHelper bugzillas.