To get links working on my Solaris laptop, I logged into JDS/GNOME then set the default applications for web and mail. I also selected the option to 'open in a terminal'. I've not experimented extensively, but that definitely worked for me, even though I use WindowMaker as my window manager.
In fact, here's a better way...
Create a file user.js in your Thunderbird prefs directory. Mine is here :-
/export/home/gjl/.thunderbird/07j4trhv.default/
In that file put the following lines (well something similar anyway, depending on your path)
user_pref("network.protocol-handler.app.https","/usr/local/firefox/firefox");
user_pref("network.protocol-handler.app.http","/usr/local/firefox/firefox");
user_pref("network.protocol-handler.app.ftp","/usr/local/firefox/firefox");
Then restart Thunderbird.
Just ran into this problem on my new Ultra 20.
The best way to make this work is to use the about:config settings. If you are running a version before 1.5, get the extension.
about:config --> network.protocol-handler.app.http
set the string to firefox "%s" (or whatever browser you use)
Do the same for ftp and https
This will open the link in a new tab if you have Firefox configured right.
If you want to make it a global setting,
Add these lines:
pref("network.protocol-handler.app.ftp","/usr/local/firefox-1.0.6/bin/firefox-wrapper");
pref("network.protocol-handler.app.http", "/usr/local/firefox-1.0.6/bin/firefox-wrapper");
pref("network.protocol-handler.app.https", "/usr/local/firefox-1.0.6/bin/firefox-wrapper");
to
/usr/local/thunderbird/lib/thunderbird/default/pref/<yourcustompref>.js