You can pretty much ignore the 'memory free' value Vista reports since it uses RAM differently than XP. To improve performance, and as well as it damned should, it uses as much memory as it pleases to go off and pre-load as much gear as it can based on your usage patterns, so that when you ask for things like commonly used applications or files they're already in memory and load quicker. When something else wants memory (everything takes priority over the cache) the cached stuff in memory gets forced out.
Looked at
http://kb.mozillazine.org/Firefox_hangs yet? Or try FF3 beta I guess. But check that post first, there's a lot of stuff in there that might apply (proxies, JavaScript). Quick test would be to throw the NoScript add-on into your install and see if it takes the edge off.