-KA-,
I'm just corious...how much mem does Opera use?
I currently use FF and it's mem usage is between 160,000 K - 380,000 K.
I just checked and I'm using a bit over 81,000 k. Have you upgraded to v. 3?
Firefox's main problem over the years has been its memory usage. Ver. 3 was supposed to improve that (and it appears they did, at least on my machine).
How many Extensions are you using (they will increase memory usage)? I'm using Adblock Plus, NoScript, Forecast Fox and Grease Monkey.
You may want to uninstall FF and reinstall. See if that makes any difference. Lastly, how long has it been since you reformatted and reinstalled? That would make a difference.