You keep on opening the same webpages again and again, the I can easily fill up the memory to 400 MB.... and that's when a single blank tab is opened. Obviously this's not an upstream bug. It has to be one of those patches. Reproducible: Always
Looks like someone else is having this problem too. Although I don't use Firefox anymore, it'll be useful to debug and help Mozilla.
Could you try with Firefox 5.0 and see if you can reproduce the problem? Also, do try with a blank profile to make sure it isn't a profile-specific problem.
Do we have any live ebuilds?