MarsSeed [1] filed a deletion request for firefox-nightly-hg-i18n [2]: Unneeded localisation addon for the orphan and defunct firefox- nightly-hg package from 2021, which has wrong dependencies now and is pending deletion. (PRQ#46739).
There is a maintained firefox-hg and firefox-wayland-hg on AUR. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/firefox-nightly-hg-i18n/
