Are you clicking the "Remember Me" thing anywhere? I know this might be obvious, but I havent signed out since I started here. xD I'm just wondering. I dont know much about Firefox, as I tend not to use it myself. I refer to it as "Failfox" myself, and I'm usually one for Internet Explorer. : /
I've never had this instance before, I can't help you more than that I'm afraid, sorry. ;_; Maybe a browser setting or something?