have you tried opening your printers in windows (devices and printers on the start menu) and actually setting that as the default?

Yes, I have set my default in devices and printers to letter size, and it holds there as such. The only problem is with Thunderbird. I called Brother support and they said the problem is my email program.

Use the Config Editor to delete the entries in the config editor starting with printer. that look like they describe a page size or dimensions. Or all settings starting with printer. They will be recreated next time you use the printer in Thunderbird as required so you can not damage anything removing printer entries.

Thanks for the suggestion Matt; you may be getting warmer, but still doesn't hold. I looked at the config in all other printers (ex. Adobe, HP Laser) and made the Brother printer the same by adding ".print_paper_size" which was not showing under Brother, but was on the others. I closed Thunderbird and reopened, but still shows "user defined" and 3x5 size. Was afraid to delete any of the Brother print settings, so just compared and added what appeared to be missing. I only had "_size_type and _unit, but not just _size. Any more thoughts?

You replied to a similar post on 4/24/14 to Isldes, who didn't seem to solve his problem either. Do you recall if he ever got it working?

I reply to hundreds of posts here each month. Most of the time I have no idea who or what 5 seconds after I am done.

I did not say add... I said delete. You can delete everything in printers and when you restart Thunderbird and try and print with the selected printer is will add the stuff as needed. So bite the bullet and just delete away.

Matt, your'e the best! It finally worked by going into the Email Config Editor, and deleted all Brother "print_paper" settings. For a while, I could not figure out how to delete them... right clicked and hit delete, but wouldn't work. Finally I tried clicking on "RESET" and it minimized the working and grayed out; closed the program and reopened; printed... and presto.... all printing again on 8 1/2 x 11, not 3 x 5 as before. Thanks for all the time you spent on this and your expertise. GREAT JOB.