<p><pre><code>  [...] Thunderbird, the free email, chat and news client 
  it first developed in 2004 [...]
</code></pre>
In 2004 Thunderbird spun off the Mozilla Suite (Browser, Mail, WYSIWYG HTML editor, IRC-chat client), like Firefox.<p>And Mozilla Suite is based itself on Netscape Suite, that AOL donated as open source code and formed the Mozilla Foundation (Mozilla was the internal codename of Netscape).<p>The Netscape Suite was a complete rewrite of Netscape Navigator 4. That long rewrite phase and the choosen technology (XUL, XPCOM, etc) contributed to loosing the Browser war agaist IE 5&#x2F;6.<p>Thunderbird barely changed since 2004. Back then it was a direct competitor to Outlook Express.