All web browsers have semi hidden pages like this for all sorts of purposes. Its not really intended to be secret, its just not stuff worth even adding to a file menu. Some of the about: pages in firefox are in some submenus, some on settings, but def not all. Tho you'll fimd them mentioned and linked in support guides.
If anything, the ability to access these is better than them being blocked...
So the site should just... Not work in firefox then?
A lot of the sites in the about:compat block or don't work in Firefox because the sites don't follow web standards