It's actually easier than what the document shows you: Once you get to the Debian shell do apt-get install firefox-esr instead of installing the flatpack. I assume they don't tell you that, because then you end up with the Debian build of Firefox instead of the Mozilla's
I am said maintainer. Note that's not part of my work at Mozilla, and that doesn't make the Debian package endorsed by Mozilla as the way to get Firefox on Debian.