This is a years-long problem through several versions of TBird and through three printers. Thunderbird keeps defaulting to my Brother MFC-9340 network (ethernet) printer regardless of the fact that Bullzip is the default printer. (Bullzip is a PDF printer). I also had the same problem with my HP5 years ago and another HP Laserjet (I think a 4000). I uninstalled the Brother and reinstalled and that produces no change. All other programs use Bullzip by default. I have also uninstalled and reinstalled TBird with no change. This happens even if I am printing the same document two times in a row without closing the mail. I have to re-select Bullzip every time.