These resources cover the basics of creating a Firefox sidebar extension:
http://developer.mozilla.org/en/docs/Creating_a_Firefox_sidebar
http://occidopagus.nl/firefox/emptysidebar/
http://kb.mozillazine.org//Creating_sidebar

Compatibility: Thunderbird 1.5 to Thunderbird 2.0.
It will probably work in Thunderbird 3.0a but it's untested in this environment.

Version 0.4.unstable (contains the latest updates):
xsidebarTIG-0.4.unstable.xpi

What's new:

• [fix] On startup sometimes the sidebar would come up hidden but with a visible splitter.

Version 0.4 (2007-08-02):
xsidebarTIG-0.4.xpi

What's new:

• [updated] Adapt more slim styles from userstyles.org for the Add-ons sidebar.
• [new] When the contacts sidebar is in messenger, hide non functioning context menu items and buttons.
• [new] When the contacts sidebar is in messenger, add context menu items to open the compose window with the highlighted address in the to, cc, or bcc field.
• [fix] fixed bustage on trunk due to the landing of the new tabbed interface.
• [new] Added latest Babelzilla translations.

Version 0.3 (2007-03-20):
xsidebarTIG-0.3.xpi

Version 0.2 (2007-01-04):
xsidebarTIG-0.2.xpi

Version 0.1 (2006-12-23) initial release:
xsidebarTIG-0.1.xpi

• [updated] Adapt more slim styles from userstyles.org for the Add-ons sidebar.
• [new] When the contacts sidebar is in messenger, hide non functioning context menu items and buttons.
• [new] When the contacts sidebar is in messenger, add context menu items to open the compose window with the highlighted address in the to, cc, or bcc field.
• [fix] fixed bustage on trunk due to the landing of the tabbed interface.
• [new] Added latest Babelzilla translations.

2007-03-20: xSidebarTIG v0.3:

• [fix] For *nix platforms change the toggleSidebar hotkey from F9 to F4.
• [new] Added a toolbarbutton for the Contacts sidebar.
• [fix] Fix load, unload and js errors in abContactsPanel.
• [new] Added sidebar-picker toolbar button to the sidebar.
• [new] Override Contacts Sidebar in message compose window.
• [fix] Improve compatibility with Contactssidebar Extension.

2007-01-04: xSidebarTIG v0.2:
[New] Extension and Theme managers (Thunderbird 1.5), and Addons manager (Thunderbird 2.0) in the sidebar.

2006-12-23: Initial release v0.1:
Based on the xSidebar code contributed to the sidebar in Moji 0.9a3.