Mozilla Thunderbird is a redesign of the Mozilla mail component. Our goal is to produce a cross platform stand alone mail application using the XUL user interface language. Our intended customer is someone who uses Mozilla Firefox (or another stand alone browser) as their primary browser and wants a mail client based on mozilla that "plays nice" with the browser. In addition, by focusing solely on stand alone mail, we believe we can make some dents in the overall footprint and performance of the mail client by removing components and chrome we don't need. On top of that, the UI becomes much cleaner as a stand alone application as opposed to being part of the mozilla suite.
In addition to the feature set found in Mozilla Mail, Mozilla Thunderbird has several new features and improvements to make your mail and new experience better.
The ability to customize your toolbars the way you want them. Choose View / Toolbars / Customize inside any window.
UI extensions can be added to Mozilla Thunderbird to customize your experience with specific features and enhancements that you need. Support for extensions.
Extensions allow you to add features particular to your needs such as offline mail support.
A new look and feel. Thunderbird also supports a large number of downloadable themes which alter the appearance of the client.
An addressing sidebar for mail compose which makes it easy and convenient to add address book contacts to emails.
Online help includes a FAQ, tips and tricks and other useful information.