Hmm, this error is so odd! We've established the following:
> Cookies are enabled
> Cookies are not limited
> Cookies are not being destroyed/lost
> It's not the browser (probably)
The only thing I can figure is that it's a confliction with a plugin, or something you guys have going on your firefox, like Shuugo said earlier, at this point all we can really do is keep looking for a connection between users who are getting logged out.
So far the only connection is that they are using Firefox.
For reference can both of you post your User-agents, and Java versions, and if you have time maybe a list of plugins you may or may not have installed on Firefox?
For help getting that information see this thread (
https://www.fakku.net/viewtopic.php?t=24546) as for plugins you should be aware which you have installed, if not just check your Plugins option in Tools.