Following the footstep of Firefox, Thunderbird had been jumping forward updates from Thunderbird 3.1 to Thunderbird 5. It has more responsive and faster to start up and use, and its based on the new Mozilla Gecko 5 engine. Even has the new Add-ons Manager, Tabs can now be reordered and dragged to different windows, Attachment sizes now displayed along with attachments and over 390 platform fixes that improve speed, performance, stability and security.
Download Thunderbird 5 at https://www.mozilla.org/en-US/thunderbird/
Installation or Upgrade for Ubuntu
Mozilla Thunderbird 5 also available on PPA, you can install or Upgrade to this latest release via the following PPA by typing following on your terminal:
$ sudo add-apt-repository ppa:mozillateam/thunderbird-stable
$ sudo apt-get update
$ sudo apt-get install thunderbird