For Ubuntu 18.04 LTS the solution is as follows.

1. System Settings > Devices > Printers: A list of your printers should appear.
2. For the selected printer click the gear wheel icon.
3. Select Printing Options > Paper Source
4. Choose default paper source from the drop down menu

For travel I set my portable printer as default in the laptop. All programs including Thundderbird then used it. When home again I set my main laser printer ad default again. All programs EXCEPT Thunderbird went back to it. In Thunderbird when starting a print I have to remember to chenge the printer selection each time. I find no reason it does not use the system default, and nowhere in Thunderbird to select its default separately. If I overlook this extra step for each printing, the jobs end up stacked in the travel printer and I have to start over to print in the actual default printer. All other programs (Word, XL, Maps etc follow the printer default set in Windows 10 settings, but not Thunderbird.

TB should use the system default printer if print_printer is Reset:

https://support.mozilla.org/en-US/questions/1165617

but there may be other 'corruptions' in the profile that prevent this from working properly. Creating a new profile from Help/Troubleshooting, about:profiles, may restore the expected behaviour.

I got it, it seems to be working. Note, a little confusing because that long list does not carry a PREFERENCE print_printer but at the end of the prefs selections was a ong list of print-printer and the last one there had the wring printer in last column. So I went ahead there and right clicked and selected RESET, and a box popped up with the wring printer in it. I deleted that and replaced it with "RESET" and after restarting it appears to be corrected. Now this a little strange requirement because last year traveling it did follow the system default without any trouble. Wonder if the new update this year missed that. Anyway Thanks, I think it is solved.