I'm trying to use Thunderbird again, but I can't make it use Firefox to open links. I'm using KDE and Firefox is my default browser. I tried the network.protocol-handler method but it doesn't work.
It seems that thunderbird uses sensible-browser, that actually tests for the following alternatives: gnome-www-browser and x-www-browser; so what you need to do is manage your altenatives with:
sudo update-alternatives --config gnome-www-browser sudo update-alternatives --config x-www-browser
And the choose Firefox for those.
Probably you'd only need to update x-www-browser since you're using KDE.