After creating the Firebird browser, the Mozilla people also wanted a separate mail client, and a theme to match. Of course, you can use Thunderbird with any other browser as well.
History
Thunderbird 0.1
First version.
Icons for main window and write message window.
Thunderbird 0.2
Icons for the options dialog box.
Improved toolbar icons.
Thunderbird 0.4
Lots of new icons for folder and thread panes.
Icons for address book toolbar and small address book related icons.
Icon for mark button in main window.
Icons for cut, copy, and paste in write message window.
Large toolbar icons are 30 × 30, previously 32 × 32.