The Firefox "Memory Leak" That’s Really a Feature (Or Vice Versa)

According to Inside Firefox - The Inside Track on Firefox Development:

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

Bottom line: setting browser.sessionhistory.max_total_viewers to something low will help avoid this memory hogging.