"First Vulnerability for Firefox 1.5 (released version) Announced - PoC available"
Packetstorm Security has released proof of concept code that causes a buffer overflow and denial of service on the Firefox browser. Long and short of it is, history.dat stores various pieces of information on websites you've visited. If the topic of a page is crafted to be long enough, it will crash the browser each time it is started after going to such a page. This vulnerability has been tested and does work, and no known patches are available at this time. Once this happens, firefox will be unable to be started until you erase the history.dat file manually.
I have just sent a link to Mozilla.org profile location documentation to help the history.dat deleting process if there is need to test a PoC. This link is located at Internet Storm Center's Diary page.