I’m having the same issue for the last few weeks, running Firefox 3.6.12, Chrome 7 and Safari 3.1.2 on Win XP SP3. It’s not happening on localhost, only on my test server and the client’s server (both with Fatcow) so it definitely seems like a server thing.
Occasionally it’s OK using Chrome on the Fatcow sites. I’ve also found that using the solution suggested by t34 does appear to have logged out but when I hit the back button I’m still logged in. I’d very much like to find a solution to this.