If Thunderbird recipient stays blank, changing sender removes content
If you face weird problems in Thunderbird, such as the email recipient field appearing blank, although you just entered an address, or if your message disappears when you try to use a different sender address, or if your mouse cursor goes immediately to the message body after you select a sender address, etc., do not panic!
Check you addons and most probably you will find there one called “Stationary”.
Stationary is a useful addon that enhances Thunderbird with mail templates, however, it produces also these extremely annoying side effects.
Before these bugs are fixed, disable Stationary, if you don’t really need it, and enjoy!