Here's a short HOWTO on Habeas headers in Mozilla.

You need to make a manual change to preferences. You can edit your user.js file (strongly suggested) or visit the URL about:config (A mozilla bug prevents that link from working; about: about:cache too).
You should definitely read up on how to edit your preferences manually quickly, so you don't shoot yourself in the foot.

You'll want to do something like this on Windows: Find your user.js file and
C:\WINDOWS\system32\notepad.exe "C:\Documents and Settings\<username>\Application Data\Mozilla\Profiles\default\<random>.slt\user.js"

Insert:

user_pref("mail.identity.default.header.habeas1", "X-Habeas-SWE-1: winter into spring");
user_pref("mail.identity.default.header.habeas2", "X-Habeas-SWE-2: brightly anticipated");
user_pref("mail.identity.default.header.habeas3", "X-Habeas-SWE-3: like Habeas SWE (tm)");
user_pref("mail.identity.default.header.habeas4", "X-Habeas-SWE-4: Copyright 2002 Habeas (tm)");
user_pref("mail.identity.default.header.habeas5", "X-Habeas-SWE-5: Sender Warranted Email (SWE) (tm). The sender of this");
user_pref("mail.identity.default.header.habeas6", "X-Habeas-SWE-6: email in exchange for a license for this Habeas");
user_pref("mail.identity.default.header.habeas7", "X-Habeas-SWE-7: warrant mark warrants that this is a Habeas Compliant");
user_pref("mail.identity.default.header.habeas8", "X-Habeas-SWE-8: Message (HCM) and not spam. Please report use of this");
user_pref("mail.identity.default.header.habeas9", "X-Habeas-SWE-9: mark in spam to <http://www.habeas.com/report/>.");
user_pref("mail.identity.default.headers", "habeas1,habeas2,habeas3,habeas4,habeas5,habeas6,habeas7,habeas8,habeas9");

To get Mozilla to notice habeas headers on incoming email, one can create a Message Filter that will match against a custom header. (Customize: add "X-Habeas-SWE-3")...

This should work in the current version of Thunderbird and the next version of Netscape. Please email me to confirm or refute this, or to say thanks.

-- Matthew Elvey