Thunderbird & Firefox

[sunrat]

I don't know Cinnamon either. Mainly I use KDE but sometimes Xfce.
It would be worth running this (as root) to see if Firefox is set:

Code: Select all

update-alternatives --config x-www-browser

or there's a GUI version called galternatives.
I had a similar problem where Chrome would steal default from Firefox no matter how I set it. Never did get it fixed but ended up removing Chrome and installing Vivaldi instead. I prefer Vivaldi anyway.