The issue with the firefox-esr-mobile-config package is that it is really outdated and does not support current Firefox ESR in PureOS Byzantium. There’s an old, pending MR on Purism’s gitlab, but that MR/ the version of the mobile-config-firefox project is also outdated by now.
I just recommend you to install postmarketOS / mobile-config-firefox · GitLab instead.
If you have git installed, you can install these customizations by running:
git clone https://gitlab.postmarketos.org/postmarketOS/mobile-config-firefox.git
cd mobile-config-firefox
make
sudo make FIREFOX_DIR=/usr/lib/firefox-esr install
You can also replace the first step by downloading the lastest tar ball from https://gitlab.postmarketos.org/postmarketOS/mobile-config-firefox/-/tags, unpacking that, and then continue with step two.