Mozilla released Mozilla Thunderbird 5 based on the Mozilla Gecko 5 engine. The new Thunderbird delivers a faster and safer email experience. It comes with a handful of new features such as drag and drop enabled interface, improved add-on manager and account creating wizard and several fixes. The most apparent change you shall notice is definitely the better, minimal and more modern interface. The Add-on manager, Trouble shooting page and many bug fixes!
Thunderbird has improved the speed of launch. It would launch in a matter of seconds now! The Add-on manager is enhanced. It will open in a new Tab. With the Add-on manager you can also customize the email client and install themes.
You will also be able to drag and drop windows, just like in the browser to make them separate windows. You will also be able to re-order the tab. Now you will also be able to view details of attachment i.e. the size of the attachment, included in the email. Plug-ins can now be loaded with RSS feeds by default.