I have been seeing an issue accessing gmail from firefox over the past few months. This started with iceweasel/firefox3, and has continued into the firefox4 series.
I have a workstation and a laptop, both running sid, both up to date. Back in the firefox3 days, I posted a thread (http://lists.debian.org/debian-user/2010/09/msg00542.html) about firefox3 leaking memory like a sieve.
Back in those days, I noticed a problem on the laptop using iceweasel 3.5.x on the laptop. Sometimes, gmail's ajax interface would load fine, and at other times it would time out, but I could get to the html one. On the workstation, I could get to either all of the time, running either iceweasel or swiftfox.
Since I have installed iceweasel4, the laptop consistently loads the gmail ajax interface, and the workstation consistently does not.