Try deleting cookies and cache:

1. Tools| Clear recent history

2. Time range to clear: Everything

3. If it isn't already selected, select __Cookies__ and __Cache__

4. __Clear now__

Also, do you get this problem in Diagnose Firefox issues using Troubleshoot Mode?

• This actually occurs due to slow internet connection. At times the internet

speed goes down and during those scenarios you will get this message.

    *You will get an option to choose the basic HTML view and once you click
on it it will reload the page and you will be able to access the email. This is a
work around.(pic 2-gmail error)
   
     *In case if the problem occurs very often you can set Basic HTML as 

default,so that you dont need to do it everytime you login to gmail.(pic 1-default)

      *Also have a check with your internet service provider if u are paying for
a high speed connection but encountering this problem often because high 

speed internet will not give this error so often.

       P.S: Check the screenshots below if u need help.

                                                                               Regards...