Ever had this when signing in to gmail using FireFox — "Your session has expired. Please sign in again" ? Would seem the way to fix it is to go into your cookies (Tools->Options->Privacy->Cookies->View cookies) and kill all the www.google.com cookies. Then try logging in again, from the www.gmail.com address, and all should be well. Seems an errant cookie can sometimes end up in your cookie jar which confuses gmail.
If you’re not sure what I’m talking about, hopefully these images’ll help. After going to the menu bar and choosing ‘Tools’ then ‘Options…’ you should get this: Click ‘Privacy’ then ‘View Cookies’ — you should get another box popping up like this: On this, scroll all the way down to the ‘www.google.com‘ cookies (in the screenshot above there’s only one; but when gmail was broken for me I had about 8-9 google cookies). Select each one in turn, and then hit ‘Remove Cookie’. You should end up with no google cookies listed. Then just hit OK on both popups and try logging back into gmail.