BCC (Blind Carbon Copy) is such a great feature in email applications.  I love to be able to send messages to various contacts who probably do not want their email address shown to every one. 

In Thunderbird one thing you can do when composing, replying, or forwarding an email is add a Bcc or Cc contact simply by clicking the To field and choosing Bcc from the drop down list.  *Keyboard Ninja Tip:  With the original message open hit Ctrl + R to reply click on the To field and Ctrl + B to change to Bcc.

The Thunderbird Plug in Contacts Add BCC button also comes in handy.  First download the extension to your hard drive.  Then in Thunderbird go to Tools Addons and click the Install button.

Windows Explorer will open where you can browse to the location of the downloaded Plug-in.  You should get a short countdown then click Install Now.

After successful installation go ahead and restart Thunderbird.

You will then have the Add to Bcc button showing in your Contacts list.