There is a big variety of browsers there, and as I said, the UA is simply one of the many tracking points.
Websites often dont support users, they live off ads because we didnt find any internet model that can live without ads, which are a horrible concept.
I was in a supermarket today where card payment was broken. There was a huge sign in the middle of the entrance about that, but a lot of people still didnt read it and had to bring back their groceries.
I think this is in part due to ads. Ads train us to not concentrate, zoom out and be passive. Otherwise, looking at all that manipulative garbage would make us insane.
So I am curious to why Websites would need to support users. Normal web standards work the same. There is a trend towards not supporting Firefox or maybe platforms with worse DRM, like Linux (where you can screencast any DRM browser anyways). So I think Netflix uses the Linux user agent to limit you to 1080p (as a laptop user and pirate I have no idea how this is an issue though)
You are lucky here with your generic UA. maybe this is also outdated, but I read this was a thing at least on some distro packaged Firefoxes.
Also that the search engines preconfigured would always get the info about what OS you are using.
Yes these things may not be critical, and Firefox does a ton of awesome things like cookie isolation and containers, to limit the creepy stuff.
But
having HTTPS off by default
being on “default” privacy level
keeping all cookies
Is simply not okay. HTTP is still possible, I only know a single popup-ads-riddled site that doesnt work with Firefoxes most private setting. And deleting all cookies and making exceptions work kinda fine.
There was a button to save cookies for a site, but it is gone? No idea why.
Improving good private UX helps. Being too shy to implement it harms its reputation I think.
I also like Brave with their model for monetization via crypto. I would be happy to tip a few cents for every website click, but micro transactions just dont really work.
But as a sensitive person, I will absolutely always block every ad possible, as ads are horrible.