There is not real indication of where the error is. However today I'm finding people that think that these machines and softwares should be trouble free and run for hours. What you could do is close all instances of the browser every 2 hours and if you found it only runs for 3 hours then you would side step the issue.
-> Another thought. When I can't end task a program I find I can log out and force the issue.
Logging out and back in is faster for me than a reboot.
I also notice you are running the SP2 which I thought was still beta. That means there can be bugs. http://tech.slashdot.org/article.pl?sid=09%2F01%2F20%2F2028250&from=rss and other posts confirm it's not out yet.
I'm using windows vista service pack 2. And I found that I often got my firefox crashed after a few hours. I mean, it can't open any website and the status is "stop". When i try to "kill" it using task manager under process tab...I can't "end process" firefox. I'm using version 3.03. Any idea what I should do?