[Gabrys1]

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

[pabs3]

The Debian package of Firefox is maintained by a Mozilla employee :)

[glandium]

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.

[Gabrys1]

Thank you for maintaining the deb!

[pabs3]

Sorry for assuming both of these, it wasn't clear that they aren't true.