On Debian based systems, the default browser is set using the alternatives mechanism.
Get browser categories
# update-alternatives --get-selections | grep browser
gnome-www-browser auto /usr/bin/google-chrome-stable
x-www-browser auto /usr/bin/google-chrome-stable
Get registered alternatives
# update-alternatives --list gnome-www-browser
/usr/bin/firefox-esr
/usr/bin/google-chrome-stable
# update-alternatives --list x-www-browser
/usr/bin/firefox-esr
/usr/bin/google-chrome-stable
/usr/bin/konqueror
Set your preferred web browser
# update-alternatives --set x-www-browser /usr/bin/firefox-esr
update-alternatives: using /usr/bin/firefox-esr to provide /usr/bin/x-www-browser (x-www-browser) in manual mode
# update-alternatives --set gnome-www-browser /usr/bin/firefox-esr
update-alternatives: using /usr/bin/firefox-esr to provide /usr/bin/gnome-www-browser (gnome-www-browser) in manual mode
Success
All apps should now use your default web browser, as long as you haven't set the browser to use explicitly in the app's settings.
Happy browsing!
