It's totally RSS related, or at least related to using Firefox for both browsing and RSS feeds.
My work computer is turned off right now, so it's not refreshing anything, and my auto-login kept getting logged out even after your last post.
When I removed the forum from Live Bookmarks earlier, my browser stayed logged in for over an hour. Just now I added the forum back to Live Bookmarks, and sure enough I was logged out promptly.
I suppose I only really need one or the other, since I use the auto-login primarily to quickly see if there are new posts (i.e. orange page icons), but it just seems so odd that one would affect the other.