Although you certainly won't be having a huge list of cool new features and if you don't like to use a beta channel then you should just stick to the stable release. But for some reason they've skipped the version "4" and jumped to the "5" instead and it does bring few improvements nonetheless.
*. New addon and extension manager API (when they release it officially they'll be changing the GUI as well).
*. Easily drag tabs from one window to the other.
*. Enhanced setting up wizard.
|Image via: ubuntuforums|
*. Attached file sizes are shown alongside them.
*. Better troubleshooting.
*. Load plugins within RSS Feeds.
*. And few other bug fixes and minor improvements.
You can install Thunderbird 5.0 beta in Ubuntu 11.04 Natty Narwhal, 10.10, 10.04 or in the upcoming Ubuntu 11.10 Alpha by entering the below command in your Terminal (we're using the Mozilla's official PPA channel).
sudo add-apt-repository ppa:mozillateam/thunderbird-next
sudo apt-get update && sudo apt-get install thunderbirdThat should do it.