In Firefox Talktalk mail shows "You have undefined new mails in your inbox" and I can't access my in box - OK if i'm in Internet Explorer.
Apparently TalkTalk mail will work with Firefox 4 and Firefox 5 if you set the browser to lie to TalkTalk mail by saying that you are still using version 3.6.
You can find the instructions for that in a post on the TalkTalk forums: Firefox 4 & TalkTalk webmail - Forums.
Note: when you open about:config the first time, it will display a warning message that you should be careful with the changes you make. That's definitely good to bear in mind.
Does it work?