Comment 9 for bug 1983202

That's weird indeed. Can you try the following?

 1. Close thunderbird, ensure it's not running any longer
 2. Hand-edit that file (/home/tony/.thunderbird/mmhujxko.default/handlers.json) to change the value of the action for the https scheme from 2 to 4
 3. Launch thunderbird and verify whether https links now open as expected in your default browser