Like with ungoogled-chromium, firefox-esr is now officially in the Universe repo. You need to add the following to your /etc/pacman.conf
[universe]
Server = https://universe.artixlinux.org/$arch
For the curious, yes the build is exactly the same as the firefox-esr AUR package (https://aur.archlinux.org/packages/firefox-esr/) so it's an optimized build and has all the usual arch compile options.
As of version 91.8.0, firefox-esr no longer conflicts with firefox and has its own profile in a separate directory (~/.mozilla/firefox-esr). On an update, you will have to copy/paste your profile from the firefox folder. Additionally, the /usr/bin/firefox binary is no longer provided by this package. You now need to specifically invoke /usr/bin/firefox-esr if you were not already.