Flathub: https://flathub.org/apps/org.garudalinux.firedragon
Browser source code: https://gitlab.com/garuda-linux/firedragon/builder
https://github.com/flathub/org.mozilla.firefox.BaseApp
https://searchfox.org/mozilla-central/source/taskcluster/docker/firefox-flatpak
flatpak install flathub org.garudalinux.firedragon
flatpak-builder build org.garudalinux.firedragon.yml
To test:
flatpak-builder --run build org.garudalinux.firedragon.yml firedragon