polyzen [1] filed a deletion request for firefox-refcontrol [2]:

Doesn't have multiprocess shims -- not sure if this is still installable. Hasn't been developed in years.

WebExtension alternate: https://aur.archlinux.org/packages/firefox-referer-control/

[1] https://aur.archlinux.org/account/polyzen/ [2] https://aur.archlinux.org/pkgbase/firefox-refcontrol/