Greetings LordB
1) To start, restart the program with disabled additions and check the settings in your account Gmail.
2) If all is well, but the problem persists - then try to re-create your profile Gmail. That should work!
3) If the problem persists after this - check the anti-virus software - there can be blocked access port.

I hope you will help it to solve the problem
Yours faithfully, Anticisco

https://support.mozilla.org/en-US/kb/cannot-receive-messages

Thanks for the responses guys but its still not working. I've got 2 other emails (from different ISP's) running and they are sending and receiving emails fine.

I'm completely baffled.

Problem solved.

I'm not sure how it happened but my connection to the server was set to use a proxy. I've changed it back to no proxy and it's all working fine now.

Thanks for the help.