Firefox ESR (on Byzantium) has since been updated to 91.6, so yay!
Sadly, this means that the old firefox-esr-mobile-config package is now entirely dysfunctional.
If you want to use Firefox on your Librem 5, you can do what did:
git clone https://gitlab.com/postmarketOS/mobile-config-firefox
cd mobile-config-firefox
make
sudo make install
You’ll need to have git and build-essential installed to do this. If you want to know what the result will look like beforehand, check out my toot.