I had to reconfigure my default printer settings in Configure Desktop >
Printers (root privileges) using KDE on openSUSE.
Then changed it in Thunderbird, quit, restarted, and it stuck on A4.
Don't know the procedure on Lubuntu, and don't know why it sticks in
Firefox, but not Thunderbird.
Now to switch it back to US Letter. Yep, change stuck.
HTH
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openSUSE 12.3 (64-bit) KDE 4.10.2
Thunderbird Release
I despise GG.