Thanks Fotix for this post in the forums. 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 Firebird (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.
Since this is our first milestone release, everything is new! In addition to the feature set found in Mozilla Mail, Thunderbird has several new features and improvements to make your mail and new experience better.