[eluser]Giords[/eluser]
I think I had the same problem and it was driving me crazy. I investigated a bit more, and found that it was, in fact, related to FirePHP (not FireBug itself).
In my config file, I had sess_match_useragent set to true. FirePhp changes the useragent, so for example it goes from:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20100101 Firefox/15.0.1
to:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:15.0) Gecko/20100101 Firefox/15.0.1 FirePHP/0.7.1
which would invalidate the session.
It would also happen if you had FirePHP active to test your website, then started using a different tab where FirePHP was not active, and while doing that your website did an ajax request or something.
Really annoying, but if you set sess_match_useragent to false, you can happily continue using FirePHP.