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.
Note: You must delete your old Mozilla Thunderbird directory rather than just overwriting the files there. Not doing so WILL result in problems and you should not file any bugs on Mozilla Thunderbird unless you've first done a clean install.

Here are some key features of "Mozilla Thunderbird":
Full-Featured Email:
· Simple to use, powerful, and customizable, Thunderbird is a full-featured email application. Thunderbird supports IMAP and POP mail protocols, as well as HTML mail formatting. Easily import your existing email accounts and messages. Built-in RSS capabilities, powerful quick search, spell check as you type, global inbox, deleting attachments and advanced message filtering round out Thunderbird's modern feature set.
Junk Mail Filters:
· Each email you receive passes through Thunderbird's leading-edge junk mail filters. Each time you mark messages as spam, Thunderbird improves its filtering so you can spend more time reading the mail that matters. Thunderbird will also use your mail provider's spam filters to keep junk mail out of your inbox.
Anti-Phishing Protection:
· Thunderbird protects you from increasingly common email scams "also known as “phishing” which try to fool you into handing over your passwords and other personal information. Thunderbird will tell you when it thinks a message might be a scam.
Advanced Security:
· Thunderbird provides enterprise and government grade security features including S/MIME, digital signing, message encryption, support for certificates and security devices. Attachments are never run without your express permission, protecting you from many worms and viruses.
Automatic Updates:
· The new Software Update feature makes it easy to get the latest security and feature updates to Thunderbird. Thunderbird automatically downloads these small updates in the background and prompts you when they are ready to be installed.
Customize Thunderbird:
· Select new button controls for your toolbars, install extensions to add new features, or change the look of your browser with themes the way Thunderbird looks and works is under your control.