There may be an error with server names, ports.. .etc. Have your email service provider send you or direct you to its setup page which has all those settings in black and white and you can compare it directly. Sometimes they update/change things and don't necessarily tell you. Your webmail would still work fine because the issue is not with usernames or passwords, but an email client such as Thunderbird would need to be reconfigured. Just verify for yourself. Don't take help-desk's word for it. :)

I am able to receive incoming mail if I disable SSL protection under Server Settings. When I turn it on, I'm unable to download new messages again. Any ideas?

Yeah, that sounds like your incoming settings. SSL/TLS protection is supported by Thunderbird, but it has to be available through your mail server host in order to work. If you choose those settings, your mail server will have ports and/or server names that are compatible with those settings - and are different than non-SSL settings.

In otherwords, SSL is NOT something YOU choose, it is something that your mail service provider has to provide. Check on their setup page to see what the settings are if you choose SSL server. Most of them have that option. But your current settings are most definitely non SSL.