Did you know that Ctrl-Enter in Thunderbird sends your email message immediately? If you do, you probably learned it the hard way, by sending a half-written email, like I just did. Awkward . . .
To get rid of this behavior, install the keyconfig extension. Once you have installed the extension and restarted Thunderbird, go to the Tools menu > Keyconfig. Choose "Send Now", and click the disable button.
If you installed keyconfig, but you can't find the "Send Now" option:
1) Open Thunderbird
2) Start composing a new message (in a new window)
3) In the main Thunderbird window, go to the Tools menu > Keyconfig
4) At the top of the Keyconfig window, there is a dropdown box, with one entry for each open Thunderbird window
- Write (no subject)
5) Choose the "Write . . ." window in the Keyconfig dropdown box
6) Send Now should now be listed in the Keyconfig listbox
7) Select Send Now, and click Disable