Uh! Firefox... Memory Hog By Design?
Ben Goodger claims to have the clue About the Firefox "memory leak".
Actually, it's not a leak. Nor a bug. Definitely a feature, huh? At least in Firefox 1.5!
To quote: « 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 few session history entries. This can be a lot of data.
Firefox has a preference browser.sessionhistory.max_total_viewers which by default is set to -1. When this value is set, Firefox caches pages depending on the system's memory: