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 few session history entries. This can be a lot of data. It’s a trade-off. What you get out of it is faster performance as you navigate the web.

Inside Firefox

Screenshot taking from my work machine.

I feel that 475mb of memory usage is more ridiculous than a “feature” should be. However, I do not plan on leaving Firefox anytime soon. I currently use it on Windows, Mac and Linux. Sidenote, Outlook is awful too.