Since most know that with the upcoming Ubuntu 11.10, Canonical has decided to replace Evolution with Thunderbird, so 11.10 users will be getting it by default, but for the latter version, we gotta install it manually, it's pretty easy thanks to the dedicated PPA nonetheless.
*. Based on the Mozilla's new Geko 6.0 engine (it's a kinda of a framework that lets developers develop applications that follows the web-standards with, somewhat, ease).
*. More secure, stable and importantly, as with Firefox 6 in GNU/Linux, it's a bit faster.
*. Many MS Windows oriented features and improvements (outlook e-mail importing fixes, Win7's jump list support, etc), who cares! :).
*. Few user interface enhancements.
*. Enhanced support for Ubuntu global menus... are among the main ones according to their release notes page.
Since the stable version 6 has hit the PPA (stable means the current official version) you can install Thunderbird 6.0 in Ubuntu 11.04, 10.10 and 10.04 by simply adding the PPA. For the whole procedure, please use the below commands (trying to act like a decent guy :D).
sudo add-apt-repository ppa:mozillateam/thunderbird-stableThat all I have to say about that :P. Enjoy the "most secure" version of the Thunderbird e-mail client software. Hope this was helpful dudes and duddies :).
sudo apt-get update
sudo apt-get install thunderbird