Firefox as memory hog (with memory leaks)

I finally killed Firefox when it had ballooned to over 400K. I restarted the session, with the same tabs, and the size had shrunk to 100K. I hate Firefox, but not as much as I hate the alternatives. I do end up using IE7 for sites that will not work with Firefox. After a small bit of usage, it has grown to 138K. How much of this is due to Yahoo Mail?