I've seen this with older versions of Netscape. If you get the solution...let us know.
I've asked this at Mozillazine, but haven't gotten any answers, yet.
I use FF 1.0 with about a dozen extensions. From time to time (but without a common cause) FF closes a second after opening. While it is not listed as an open application in task manager, the process is listed. Only after ending the process can I load FF.
Since I have a workaround, it is not crucial. I did try deleting the profile and a fresh install to no avail. This never occurred in any previous versions of FF.
I use XP Home and am very diligent in the war against parasites.