I'm on Firefox too.
I've noticed this for a while. If I'm logged in and, say, leave the screen idle for a while (can't remember how many minutes it does it after!), then I get a screen come up asking for my log-in details again. It's odd, though, as when I'm doing a thread reply/PM and I've left what I'm doing for a bit and then am asked to sign in again when I click to preview, I don't lose anything I've typed, with the text just going straight into its preview form as if I'd never got logged out in the first place!
I've been having some internet connection probs recently
, which would explain why sometimes I'm being logged out (so I'm obviously being logged out when my connection cuts out!), but I'm sure it's been happening when my connection is stable too....unless my pesky memory is playing tricks on me, that is!
And *sobs* it's only on PSF that this happens...
xx Joanna xx