Setting default printer in Thunderbird
 1.  From the Print Menu / Dialog box, select the desired printer.
2.  From the Tools drop-down menu, select Options.
3.  Select the Advanced Tab.
4.  Under Advanced, select the General Tab.
5.  At lower right, select Config Editor.
6.  Scroll down the list and find print.print_printer.
(Note:  You can type "p" to speed this up like you do when installing printers)
7.  Right click on print.print_printer.
8.  Select Reset.
9.  Close down Thunderbird after you exit Options and restart.
10.  Verify with user that the correct default printer is set.