AS example did you do this: Select an email so it opens in Message Pane Right click on email in list or Message Pane and select 'Print'

A Print preview window should open showing the email you wish to Print. Check all settings are as required. Top right where it says 'Destination' - is the printer you want to use in the drop down list ? If yes, select the printer. Then click on 'Print'

What is the printer you want to use? How many printers do you use?

What OS are you using ? If Windows OS....in search type printer - select 'printers and scanners' Is this selected : 'Allow Windows to manage my default printer' ?

To manually choose which printer you want to set as default you would first need to uncheck the 'Allow Windows to manage my default printer'

What is set up in Thunderbird: Help > Troublshooting Information Scroll down to 'Modified print settings' section for 'print_printer' what does it say ?

Hello there

In help/troubleshooting more at the bottom there is a button for settings saved and clear prints.try clicking on that and then try to restart Thunderbird and see if it is done correctly now and it just prints with the default printer of yours?

Hello there

See if starting up in safe or trouble shoot mode http://kb.mozillazine.org/Safe_Mode If add ons are interfering that process?And try to print now when in safe mode?

Hello there

https://support.mozilla.org/en-US/kb/troubleshoot-mode-thunderbird