You can try Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems as they are the likely culprit. Firefox 4 is actually very good about managing memory (for example, http://blog.unitedheroes.net/archives/p/4192 ).

Thanks for the reply.

The problem is, since it is a memory leak (a slow and intermittent problem), debugging by systematically disabling potential trouble causing addons could take a ridiculous amount of time. It takes on average 30mins - a few hours for firefox to rack up its high memory usage, and I have a dozen addons. This means that even if I was to disable them half at a time in a binary search like manner, discovering the cause of the leak could take at least half a day of constant testing. Is there a better way?

Even with Firefox 4.0 (Release) running in Safe Mode, with all add-ons and themes disabled, I'm still inclined to think there's something screwy going on here.

I was watching Page Faults/sec, Page File Bytes and Working Set in Performance Monitor and tailing the Privoxy log for requests. Even with Firefox minimized and "doing nothing" (making no requests, anyway), over the space of a 10 minute period the Working Set grew from 244,375,552 bytes to 274,939,004 bytes (averaging 50,939 bytes/second). This behaviour doesn't seem consistent though - sometimes it doesn't seem to grow at all.

Additionally the Page Faults/Sec went nuts, accompanied by a step in Page File Bytes and Working Set, whenever a request got made to http://safebrowsing.clients.google.com/safebrowsing/downloads which seems to happens on a regular basis (approximately every 30 minutes).

Workaround found: Open about:blank in one tab and the memory leak is gone. If memory still grows refresh about:blank to free memory.