MarsSeed [1] filed a deletion request for firefox-developer-hg [2]: Unneeded VCS package. The referenced source code repo is discontinued (not updated) since 2017.
Package is an orphan. Flagged since 2020. Last comment is from 2017. [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/firefox-developer-hg/
