MarsSeed [1] filed a deletion request for firefox-nightly-hg [2]: Orphan package last updated in 2021 and now its dependencies are incorrect.
But it doesn't make sense to keep this. There is a maintained firefox-hg and firefox-wayland-hg package on AUR. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/firefox-nightly-hg/
