http://xrl.us/om8a8 website loaded in Firefox 3 crashes Xfce
That's probably because it's a huge page and that causes X (not its xfce clients) to be killed along with Firefox. If you insist this is a software bug instead of a lack of hardware resources, please include all required information in this bug report, not least your `emerge --info' but perhaps also a useable backtrace of Firefox loading that URL, or better yet some process accounting logs of Firefox using up all available memory.
Also, please tell which ebuild you used to install Firefox 3 (an ordinary package atom will do nicely).