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.
In Thunderbird 5 I noticed this strange behaviour, pressing the Enter button did not enter the email address but it deleted it, while the cursor remained in the same editing box, now blank again. No matter how did I select the address — writing it, or pasting it or selecting it among Thunderbird suggestions from the Address Book, the address just disappeared when I pressed Enter.
This seems to be a bug, but it goes away letting the precious functionality of the Enter button return, if you change the theme of Thunderbird.
The default theme, strangely, 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 can not try all of them. At least Nautipolis and Silvermel gave me back the Enter button, and you can try one of these themes too, if you miss this functionality, unless you have a different solution. (If you prefer an appearance closer to the default Thunderbird theme, i.e., more transparent Vista like view, try first Nautipolis).