MRC Compose is a Thunderbird extension that modify the address panel in the compose window. It replaces the default list by standards fields, where you can add several recipients separated by comma. With that, Thunderbird behaves just like all other email software...
I wrote this extension months ago (last version is from March 30, 2013). And since, some work has been done : Andreas Wagner kindly added LDAP support ! And last week, I refactored search code to have rock-solid searches (both local and LDAP).
There are still some few tasks in the TODO list, but I hope there'll be a new version very soon.
You can check it : https://addons.mozilla.org/fr/thunderbird/addon/mrc-compose/