I recently upgraded my Ubuntu Karmic (9.10) install to use Thunderbird 3.X and Firefox 3.6.X. Somewhere in my mucking about I broke the open link in browser option. Here’s how to fix it:
Brute force method:
1) Close TB.
2) mv ~/.*thunderbird/*default*/mimeTypes.rdf ~/.*thunderbird/*default*/mimeTypes.rdf.old
3) Launch TB and click a link. TB should now ask how you want to handle http links.
4) Enter the path to FF (or the browser of your choice). Typically /usr/bin/firefox is a link to the current browser.
5) You're done.
More elegant (weapon for a more elegant time…) method:
1) Edit - Preferences - Attachments
2) http -> Use Other -> /usr/bin/firefox
3) repeate for https.