The difference is that every computer comes with a app where you can copy-paste that URL and see the full website. With RSS, you first need to find and install a 3rd party software to view it and/or create an account on a website.
This was one feature I really liked about the old Opera. I seem to recall being able to look at an RSS feed and it applied a basic style to it from the get-go, as it had baked-in RSS support.
That’s mostly because tech companies have been trying to kill open protocols like RSS in order to lock people into their proprietary platforms.
That doesn't answer why Firefox doesn't have RSS support anymore though (removed in 2018)
I don’t think Firefox continuing to support RSS would have made a difference. It makes sense for Mozilla to prioritize features that users actually want over trying to maintain some idealistic vision that was already dead.