I got it working again! If any other Thunderbird users encounter this issue, what I did was:
(1) Uninstalled Outlook by clicking on the Office installation in Add/Remove Programs and then using the Repair/Modify option
(2) Ran a registry cleaner to remove traces of Outlook
(3) Used a small, free utility called DefaultMail to set Thunderbird as default email client.
And now it works.