Thunderbird only has the ability to print emails or an address book.

Thunderbird is not the correct software for wanting to perform something like printing labels. You would need to use MSOffice package or OpenOffice.org (free) see link: https://www.openoffice.org/

Using OpenOffice.org Base you can select to connect to an existing database and select Thunderbird Address Book. After saving to a suitable location, you can then use OpenOffice.org Writer. File > New > Labels select the saved filename.odb database.

See image:

Thanks for your help. I was afraid this would be the case--seems like a serious shortcoming for a computerized address book... (BTW, you can also make a Thunderbird mailing list of the addresses you want, export it to an Excel file (.cvs), and then use Avery's "Avery Wizard" plug-in for Microsoft Office to print any of their labels (their software has a few bugs, though).)