I had the exact same problem with Thunderbird and HP Laserjet 1020 on Windows 7. It was caused by some sort of corruption in the printer driver. Apparently, HP LaserJet drivers don't like it when you have other printers connected to the same PC as well.
This is how I fixed it: Disconnect the HP printer. Delete the printer from "Devices and Printers". Restart the computer, reconnect the printer, reinstall it with fresh drivers downloaded from the HP website, and set it as default. Start Thunderbird, print something to the HP printer, restart Thunderbird, print something again and see if the default printer sticks.
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