Slashdot | Firefox Memory Leak is a Feature: "firefox's memory usage has always been a thorn in my side. I tend to average around 20 to 25 tabs open, usually while I'm running other ram hungry applications. Firefox generally was eating up about 200-250 megs of ram on my machine (and I've seen it go as high as 600 megs). After changing the browser.sessionhistory.max_total_viewers to 0 and running 'top' firefox seems to be using about 46 megs of ram right now. It also doesn't feel particularly slower than it did before. I have a feeling that the benefit of caching so much was actually having a negative return after a certain point because the machine was so starved for ram.
On a side note, if anyone is like me and looks in about:config for browser.sessionhistory.max_total_viewers and doesn't see it, you have to actually add the line. Right click and choose 'new' then type in 'browser.sessionhistory.max_total_viewers' and then 0 (or whatever you like)."