This is for the people who use BT4 R2 as their main OS (and also everybody else who's interested of course) and want to update to Thunderbird 3.1.
I found that the version BT comes with is rather old, but usable.
The n00b that I am, I couldn't figure out how to properly install the latest Thunderbird when downloaded from the mozilla website. The download contained no install files and I couldn't find a guide either (from the mozilla website I mean) and I didn't want to manually move the files to a directory to get it working.
I found a .deb file of Thunderbird 3.1 which I was able to install and now I can use perfectly.
Download Thunderbird 3.1 .deb file
I usually use gdebi to install .deb files. (I did not have any version of Thunderbird installed when I did this).
Be sure to backup you previous Thunderbird files.
When installed you can run "thunderbird" from the konsole or from the menu.
PS: I couldn't find the latest version (3.1.9) in a .deb file but if someone has a link, post a comment.