Case_Of [1] filed a deletion request for firefox-esr-gtk2 [2]: Firefox ESR is now at version 60 upstream and this new milestone does not offers the ability to build with gtk2, then this package is deprecated and should be removed from aur.
[1] https://aur.archlinux.org/account/Case_Of/ [2] https://aur.archlinux.org/pkgbase/firefox-esr-gtk2/
