Davide Bresolin has created a build of Enigmail 0.91.0 for Thunderbird 1.0.x. It is available from his homepage.
URL: http://www.dimi.uniud.it/bresolin/warpzilla
"Enigmail is an extension to the mail client of Mozilla / Netscape and Mozilla Thunderbird which allows users to access the authentication and encryption features provided by GnuPG (see screenshots). Enigmail is open source and dually-licensed under the GNU General Public License and the Mozilla Public License."
Source: OS2VOICE