My Mac keeps suspending my session and I can't return to it

Several times, when I try to return to my session after the screensaver has been on for awhile, it stalls for about 30 seconds, then the spinning wheel that you see when you turn the computer on shows up and all of a sudden I'm back at the login screen. When I log back in nothing is open anymore, but if I try to open Firefox it says it can't because a copy is already open. Does anyone know why this keeps happening and what I can do to get back to my original session, instead of opening a new one? Right now all I can do is restart so that I can open Firefox again. I really wish OS X had a switch user feature...

 

I believe it's going to sleep. I usually move the mouse around automatically, sometimes not even looking at the screen, so I can't be certain but I'm pretty sure it is.

I run HDD. I'm running 10.6.6 on a 2.2 GHz Core 2 Duo on a MacBook Pro.

 

Yes. I was wondering if it had anything to do with the update. I know the last two times have been since I updated, but I couldn't remember about the first. Has anyone had any luck when this happens to them?