Saved here so I never need to look any further, instructions for setting/resetting the default printer in Thunderbird:
This worked for me:
If you can’t print from Thunderbird, or if Thunderbird won’t recognize the computer’s default printer:
1. In Thunderbird, go to Tools – Options.
2. Under Advanced, on the General tab, click on the Config Editor button.
3. Scroll down to the line starting with print.print_printer. If you don’t have that line, then everything should already be set correctly.
4. Right-click on the print.print_printer line & choose Reset.
5. Close the configuration window, then click OK to close the Options window.
6. Close Thunderbird.
7. Make sure that the correct default printer is selected. To do so, go to Start – Settings – Printers and Faxes. The one with the small checkmark is the default. To set a different printer as the default, right-click on it & choose Set as Default Printer.
8. Reopen Thunderbird.
Note that at step 4, if you name a specific printer printer at this step, then Thunderbird will ignore whatever the system default printer is.
Advice graciously taken from a post at this forum (Ars Technica).