I prefer Kubuntu over Ubuntu, because I just love KDE, while I can't stand the "dumb-it-down-at-all-costs" philosophy of Gnome. Nonetheless, I use a lot of GTK/Gnome apps in my desktop, and two I can't live without are the Mozilla's Firefox and Thunderbird.
However, Kubuntu sets Konqueror as the default browser. Not a wise choice IMHO (Konqueror is good but not as good as Firefox for a browser; it kicks ass as a file manager, by the way) but that's it. This means that when you click an URL in Thunderbird, it will open Konqueror. And there is no obvious setting to change it.
You have two strategies to let Mozilla Thunderbird play nice with Firefox under Kubuntu/KDE. The first -and more drastic, since it changes global settings- is to change your default browser to Firefox. To do that (straight from the Mozilla Zine:
* Open kcontrol (KDE configuration center).
* Go to "Components -> Components chooser -> Web browser".
* Check "The following browser", and type in "firefox" ("mozilla" for Mozilla).
If you don't notice the "Web browser" component:
* Go to "Components -> File association -> text -> html".
* Select "Add..." under "Application Preference Order".
* Write the command firefox %U (mozilla %U for Mozilla) and select OK. You need "%U" so you can load URLs that are non-local files.
Alternatively, you can change the setting only for Thunderbird. This is a bit more obscure, but even faster to do:
* Close Thunderbird.
* Find your Thunderbird prefs.js file -from the Linux shell, try using
locate prefs.js | grep thunderbird.
* Edit the prefs.js, adding the following line at the bottom:
* Reopen Thunderbird, et voilà it should work.