Anybody know why Firefox stops remembering “remember me” logins?

It’s hardly the biggest problem in the world, I know, but it’s bugging me: every few days, Firefox (3.whatever, Intel Mac and PPC Mac versions) ignores “remember me” login cookies. I’ve checked preferences and can’t see anything. Have I missed something obvious? As the Pirate with a steering wheel down his trousers told the doctor, it’s driving me nuts.