Two things I can think of.

First, what others have said, purging one's cache/cookies/etc can sometimes fix this. It happened to me once also, but hasn't happened again.

Second, it could be browser settings. Firefox can do something called "private browsing", which automagically purges all user data when the browser is closed. This means cookies, cache, settings, url history, etc. The other part in Firefox is in the Options/Privacy tab, "clear settings" when done. If either of those are used, it could cause this behavior.

IE is a giant turd and I refuse to use it, so I can't provide any help there.