Since version 2 of Firefox I’ve noticed that it doesn’t release the memory it allocates back to the system after you close some of the open tabs. We’re on version 4 now and the memory issue is still there. I was actually running every beta and RC of version 4 and every time I checking to see if the issue was solved but no luck. I reproduced the problem both on Windows 7 and Ubuntu. To be honest, if Chrome’s development tools were as good as Firebug I’d probably have uninstalled Firefox already.