This randomly effects my box at home (XP Media Center, Firefox) when hitting NC4x4... none of the other (2 dozen) sites it stays logged into has this issue...
I checked the logs, but didn't see anything unusual. A lot of keepalive connections unexpectedly closed by the client (you), but that shouldn't cause any problems.
A quick google search turns up a lot of people blaming their routers for the problem -- in particular, a series of Linksys routers that randomly misdirect packages.
I've disabled keepalive on our side for now. If it happens again, try restarting your computer and/or your router, see if anything changes.