I am using Thunderbird 19 and the new Beta posted today and still can't get my mail. I have imported the certificate into both Firefox 19 B6 and Thunderbird. No problems with Firefox, but no mail in Thunderbird. I have a email account IMAP Gmail and a POP account from Verizon.
First, the certificates are now imported automatically to Thunderbird, sou you do not have to do it manually.
If you have imported the certificate from the previous installation to Thunderbird and did not delete it before installing the new beta, than it will not work because the installer will not overwrite the certificate with the correct (current) one. So first delete the certificate from Thunderbird and then install avast!. If you have avast already installed, it should be sufficient to delete the certificate and do a "log out/log in", the correct certificate should be automatically installed.
If you use the correct certificate, it should work. We test the mailshield with Apple Mail, Thunderbird, Sparrow and Postbox on several mail accounts (iCloud, Gmail, hotmail and some other) and have no known issues. If it still does not work, look into the system log file (/var/log/system.log), the new beta has extended logging, so there may be some useful info for us. Your IMAP account setup in Thunderbird would also be usefull for us, maybe You are using some unusual configuration.