So after a considerable amount of trial and error, I have made a PKGBUILD for the UXP repository noted by @vonPalitroque that I hope works for more than just me. As you can see by the comments I've left, I'm not totally sure what I'm doing. I pulled a few bits and pieces from old PKGBUILDs for Firefox, and cleaned it up quite a bit otherwise. Note that this requires two new patches. Also as noted in this, there is nothing magical about the commit chosen, it's just the most recent one at this time.
PKGBUILD: https://pastebin.com/CYEYR0Wv
nss_mozbuild.patch: https://pastebin.com/muFRVeUH
modified-install-dir.patch: https://pastebin.com/uEeykV0b
I've tried to document everything I did as well as possible, my apologies if I did anything dumb.
testing your changes