Are you using a bookmark for the forum?
If so, does does the url contain something like .quest3d5?
I was constantly being logged out last week and it appeared that the url in my bookmark was not exactly
quest3d.com/forum/ (it was different for some reason).
I deleted my old bookmark (with the .quest3d5 suffix) and added a new one from the home page.
I'm still logged in a week later, even after many FireFox exits.
Not sure if your problem is related, but worth a check.
My bookmark was forum.quest3d.com, if I use quest3d.com/forum, it does work, no more logging out.