~/.config/mimeapps.list
[Default Applications]
text/html=org.mozilla.firefox.desktop
x-scheme-handler/http=org.mozilla.firefox.desktop
x-scheme-handler/https=org.mozilla.firefox.desktop
x-scheme-handler/about=org.mozilla.firefox.desktop
x-scheme-handler/unknown=org.mozilla.firefox.desktop

check:
gio open http:\\www.google.com
gio mime x-scheme-handler/http
gio mime x-scheme-handler/https
    https://github.com/telegramdesktop/tdesktop/issues/24506
    https://help.gnome.org/admin/system-admin-guide/stable/mime-types-application-user.html.en
xdg-mime query default x-scheme-handler/https

xdg-open https://google.com
xdg-settings get default-web-browser
xdg-settings get default-url-scheme-handler https
xdg-settings get default-url-scheme-handler http
xdg-settings set default-url-scheme-handler https

update-alternatives --display x-www-browser
sudo update-alternatives --config x-www-browser
    ls -l /etc/alternatives/x-www-browser
    ls -l /usr/bin/x-www-browser
sudo update-alternatives --install /usr/bin/x-www-browser x-www-browser /var/lib/flatpak/exports/bin/org.mozilla.firefox 210
sudo update-alternatives --remove x-www-browser /var/lib/flatpak/exports/bin/org.mozilla.firefox 210
    https://unix.stackexchange.com/questions/584478/how-to-add-browser-to-update-alternatives

update-alternatives --display gnome-www-browser
sudo update-alternatives --config gnome-www-browser
sudo update-alternatives --install /usr/bin/gnome-www-browser gnome-www-browser /var/lib/flatpak/exports/bin/org.mozilla.firefox 210
sudo update-alternatives --remove gnome-www-browser /var/lib/flatpak/exports/bin/org.mozilla.firefox 210


2021
https://saveriomiroddi.github.io/Linux-associating-file-types-to-applications/
????
https://unix.stackexchange.com/questions/614899/setting-a-flatpak-app-as-the-default-web-browser

$ flatpak permissions desktop-used-apps


$ file --mime demo.fakeext.txt
demo.fakeext.txt: image/gif; charset=binary

$ xdg-mime query default text/markdown
code.desktop
$ xdg-mime default code.desktop text/markdown

application/epub+zip=FBReader.desktop

xdg-mime default FBReader.desktop application/epub+zip
xdg-mime default com.github.johnfactotum.Foliate.desktop application/epub+zip


xdg-settings get default-web-browser