If Thunderbird removes the recipient eMail address when you press enter
Normally in Thunderbird when you write, paste or select an email address at the recipient’s editing box (To:), then pressing the Enter button, this address is kept while the cursor moves to the next editing box letting you add another address. This way, after you finish adding recipients to your email message you can move to the subject and body of your message using the Tab key — without ever needing to leave the keyboard and use the mouse.
Sometimes you may face this strange behaviour, pressing the Enter button won’t enter the email address but removes it, while the cursor remains in the same editing box, now blank again!
This seems to be a bug, but it goes away, if you change the theme of Thunderbird!
In my case the default theme hinders the Enter! Enter did not work also with the Green theme (based on Netscape), but it did work with Nautipolis and Silvermel themes. It may also work with other themes, but I cannot try everything.