For the past 2-3 days, automatic logout is happening for my Stack Overflow account when using Firefox. It doesn't happen every time and looks to be random. I am using MyOpenID for authentication and have checked the option 'stay signed in' if it matters. Is this some sort of bug?
I suspect rogue add-ins in Firefox. No other reports of this, and we haven't experienced it on our fairly large (20+ people) team in daily use of all sites.
Try running in Firefox safe mode, or switch browsers for a few days.