This comment was just posted on my blog, and is identical to my own experience using both Firefox 3 and Portable 3
"I had the beta version and it eliminated the memory issues on all versions but then it did its upgrade to the current version and the memory problem came back. I had to delete version 2 and reinstall fresh. Not sure why, but now my version 3 still has the memory issue even after reinstalling it."
[Moved to Firefox forum by mod SL]
You probably have a memory-hungry or memory-leaking extension. I bet a good chunk of people who are complaining about Firefox's memory usage are using a memory-eating extension or theme.
And BTW wrong forum.
Signature automatically removed for being too awesome.