Ever since I began using Thunderbird as my regular mail client, ten years ago, I was annoyed by the way the window to compose a new mail was organized. To add a new recipient, one had, and still has, to click over several lines of text input. It is not practical, does not save screen space and does not allow very simple things like pasting a list of comma-separated e-mail addresses in the “To:” field.
This annoying interface problem is solved by MRC Compose, a fantastic addon for Thunderbird. The compose window looks now like this:
The interface is in French, sorry, but it shows how you can add as many recipients as you like in a single entry field. The cogwheel on the left allows adding other fields (Cc, Bcc, Fw, Reply) to be added. To have some of the fields persistent, the Preferences of the addon allow that.
Altogether, an excellent addon that does one thing really well.
PS The theme used for the screenshot is Silvermel.
PPS. Another great extension, useful if you send Word files from a Mac to people who do not use Thunderbird is no application/applefile.
UPDATE: The MRC Compose and the Conversations add-ons are the ones that I constantly use on Thunderbird. Long live open-source Thunderbird!