When you write new message and add an email address in the to field, after pressing enter, you can then press the button called Add Salutation, it then searches the address book for the first name of that email address and then adds to top of email.

Only works on Version 142+

Can now be installed on all versions of Thunderbird.
Looks for address book first name first, and if version 142+ will get first part of email address to use as name.

Only works on Thunderbird version 1.42+