Last month when i upgraded my browser to FF 3.5 I started facing the issue and realized that the issue was with Firefox and not Gmail since it was working properly in safari and other browsers. So as usual I googled and found this http://groups.google.com/group/mozilla.support.firefox/browse_thread/thread/805c8e689b64b8d2/adf482dcb49bb423

So I came to know that I am not the only one facing the issue. Then I tried couple of suggestions given in the thread and finally I have the list and solution for solving this problem. Here are the steps which you need to follow to get things working properly.

1) Clear history, cache every thing using the Clear history option in your browser

2) Next step is open GMail go to settings and check the last option Browser connection: Select Don’t always use https. Using Https will make things secure but slow. so let Gmail manage it on its own.

3) Look if you are using Firebug (A Mozilla Addon especially used by web developers) . If you are using that Disable it dont keep it running all the time.

4) Now finally the labs section. Gmail has many new features in the lab section. Try to disable the ones you don’t need.

5) Remove all unwanted add-ons

I think thats all restart Firefox and things should be alrite. If you are still facing issues Post me a comment and I shall help you out.