If the new account wizard fails to automatically detect the correct server settings use Manual Setup instead. https://support.mozilla.org/en-US/kb/manual-account-configuration

Note, you'll need to obtain the correct settings from your email provider prior to setting up the account.

Thanks, but I have settings and they appear on the page everything is automatically filled in .. do you think its because my account is a webmail account.. that is already set up and working fine on thunderbird?.. the only change will be imap instead of pop..

I have settings and they appear on the page everything is automatically filled in

I'm not sure what you're trying to say. If everything would be correct you wouldn't get an error, would you?

do you think its because my account is a webmail account.

I have no idea what a 'webmail account' is. You do have an email account with an email provider. How you do access that account, i.e. via webmail or Thunderbird doesn't matter. It is still an email account.

that is already set up and working fine on thunderbird?

You cannot change an existing account. So you need to set up the account as IMAP account in parallel to the existing POP account. http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

Thanks for the information - so it seems I can't open an imap email with the same email address as the current POP one? As I have followed the instructions to set up account using the existing email address.. which is when on the next page I get the error message? I have now tried putting in the server details which I used for my phone imap email and clicked retest, but still same error message is there!

I can only repeat what was said before. https://support.mozilla.org/en-US/questions/1105502#answer-833193