First, locate your Thunderbird directory.

With Debian, it's ~/.mozilla-thunderbird/default/xxxxxxxx.slt/

create a custom file user.js:

touch  ~/.mozilla-thunderbird/default/xxxxxxxx.slt/user.js

Then with your favourite text editor, add the following lines:

// Show the user agent of incoming messages
user_pref("mailnews.headers.showUserAgent", true);

• http://forums.mozillazine.org/viewtopic.php?p=180859#180859
• http://forums.mozillazine.org/viewtopic.php?t=16840&postdays=0&postorder=asc&start=15
• http://www.mozilla.org/projects/thunderbird/identities.html

If you are using IMAP and procmail, you want to check other folders than just Inbox.

Add in prefs.js the following line:

user_pref("mail.check_all_imap_folders_for_new", true);

• Mozilla Thunderbird Help
• An introduction to Thunderbird 9 good short articles to start with Thunderbird
• Mozilla Thunderbird: Editing Configuration Files
• Customising Mozilla
• Official forums

• Mozilla Thunderbird en français
• Thunderbird : le courrier électronique libre tutoriel de chez Framasoft

What I'd like to know is this:

• how to have the signature optional?