why is firefox 5 incompatible with talktalk email. i've been told to use another browser

problem with sending emails on talktalk. They are very very slow and I get this messaage 'The page isn't redirecting properly. Firefox has detected that the server is redirecting the request for the address in a ... (and then this message stops there) I contacted talktalk and they said Firefox 5 is incompatible with talktalk email and I should use another browser. Can I go back to Firefox 4, if not I'll have to quit Firefox Why is this problem happening