Hi,
Are you running any themes, addons, or extentions other than the defualt ones? You might want to try running FF in safe mode, this will turn everything off and only run from defualts, this will let you see if it is your connection and not the browser. If you find that you can get to gmail fine, then I would recommend that you go back to the normal version of FF and disable everything.
Then you will need to start adding one thing at a time, and see if you are having problems with any of them. I know that this is a long process but it is the only way to do it.
Cheers!