I have this problem, the remember me in Firefox simply refuses to remember for this site in particular. Damn the stupid computer, it was created to obey my every command!
This has been an issue for me since I joined this forum. Now that it's mentioned, I do check the forum from home and work and every day I am forced to sign back in HOWEVER, I did not check it from home yesterday, but did from work and it remembered me today from work... that must be it.
Yep. I guess this is one of those things everyone knows about but no one talks about. Like your parents having sex or menstruation.
Eventually, I stopped ticking the box to no apparent effect.
I've had a similar problem. Occasionally I will just suddenly be logged out for no apparent reason. Often refreshing or navigating somewhere else fixes the problem (which makes me think it might be the page didn't load all the way, or something, though I have never had this issue anywhere else), but when it doesn't I have to manually navigate to the login.form page as the login in the top left has never ever worked for me.