have you tried opening the printers in your start menu and changing the default paper for the printer to Letter? BTW my personal bugbear is printers that default to letter (designed in USA?) as the rest of us don't use Letter)

Thank you for the reply! Yes I checked the settings and the printer is defaulted to letter but for some reason Thunderbird is changing it. I had the problem w/ Safari a few weeks back and was able to adjust the paper size and it's been fine since, but all of a sudden now thunderbird is doing it.