Thunderbird 5.0 has been released today, bringing a new addons manager (Firefox-style), NotifyOSD notifications, tabs can be reordered and dragged to different windows, the account creation wizard has been improved and many other improvements and bug fixes. See the release changelog here.
Upgrade To Thunderbird 5.0 in Ubuntu 11.04, 10.10 or 10.04
By using the instructions below, your current stable Thunderbird build will be replaced with Thunderbird 5.0.
To add the Thunderbird Stable PPA and upgrade to the latest Thunderbird 5.0 in Ubuntu 11.04 Natty Narwhal, 10.10 Maverick Meerkat or 10.04 Lucid Lynx, use the commands below:
sudo add-apt-repository ppa:mozillateam/thunderbird-stable sudo apt-get update sudo apt-get upgrade
The Thunderbird Stable PPA also comes with an updated Lightning extension (1.0 beta 4 which apparently is considered a stable version - see here) which is compatible with the latest Thunderbird 5. You can install it using the command below:
sudo apt-get install xul-ext-lightning
P.s: to make Thunderbird 5.0 look like in my first screenshot (Gmail-like conversation view), use the Thunderbird Conversations extension.