http://weblogs.mozillazine.org/ben/archives/009749.html
About the Firefox "memory leak"
A lot of people complain about the Firefox "memory leak(s)". All versions of Firefox no doubt leak memory - it is a common problem with software this complicated. We look to fix the issues where we can. David Baron and others have done a huge amount of excellent work in this area.
What I think many people are talking about however with Firefox 1.5 is not really a memory leak at all. It is in fact a feature.
To improve performance when navigating (studies show that 39% of all page navigations are renavigations to pages visited < 10 pages ago, usually using the back button), Firefox 1.5 implements a Back-Forward cache that retains the rendered document for the last five session history entries for each tab. This is a lot of data. If you have a lot of tabs, Firefox's memory usage can climb dramatically. It's a trade-off. What you get out of it is faster performance as you navigate the web.
For those who remain concerned, here's how the feature works. Firefox has a preference browser.sessionhistory.max_total_viewers which by default is set to -1. When set to this value, Firefox calculates the amount of memory in the system, according to this breakdown:
RAM | Number of Cached Pages
32MB | 0
64MB | 1
128MB | 2
256MB | 3
512MB | 5
1GB | 8
2GB | 8
4GB | 8(reference: nsSHistory.cpp)
No more than 8 pages per tab are ever cached in this fashion, by default. If you set this preference to another value, e.g. 25, 25 pages will be cached for every tab. You can set it to 0 to disable the feature, but your page load performance will suffer.
