Firefox on Linux can develop the habit to (rather randomly) ask again and again to be made the default browser. E.g. when started from Thunderbird by clicking a link it asks but when starting from a shell all is fine.
The reason to this is often two (or more) .desktop entries competing with each other.
So, walkthrough: (GOTO 10 in case you are sure to have all the basics right)
update-alternatives --display x-www-browser
update-alternatives --display gnome-www-browser
should both show firefox for you. If not
update-alternatives --config <entry>
the entry to fix the preference on /usr/bin/firefox.
Check (where available)
exo-preferred-applications
that the "Internet Browser" is "Firefox".
Check (where available)
xfce4-mime-settings
that anything containing "html" points to Firefox (or is left at a non-user set default).
Check (where available)
xdg-settings get default-web-browser
that you get firefox.desktop. If not run
xdg-settings check default-web-browser firefox.desktop
If you are running Gnome, check
xdg-settings get default-url-scheme-handler http
and the same for https.
LABEL 10:
Run
sensible-editor ~/.config/mimeapps.list
and remove all entries that contain something like userapp-Firefox-<random>.desktop.
Run
find ~/.local/share/applications -iname "userapp-firefox*.desktop"
and delete these files or move them away.
Done.
Once you have it working again consider disabling the option for Firefox to check whether it is the default browser. Because it will otherwise create those pesky userapp-Firefox-<random>.desktop files again.
Configuring Linux is easy, innit?
