All of those are still standing on Firefox’s shoulders and the actual rendering engine on the browser isn’t really trivial thing to build. Sure, they’re not going away, and likely Firefox will be around too for quite a while, but the world wide web as we currently know it is changing and Google and Microsoft are few of the bigger players pushing the change.

If you’re old enough you’ll remember the banners ‘Best viewed with <this browser> on <that resolution>’, and it’s not too far off from the future we’ll have if the big players get their wishes. Things like google suite, whatever meta is offering and pretty much “the internet” as your Joe Average understands it wants to implement technology where it’s not possible to block ads or modify the content you’re shown in any other way. It’s not too far off from your online banking and other very much real life affecting services start to have boundaries in place where they require certain level of ‘security’ from your browser and you can bet that things which allow content modifying things, like adblocker, doesn’t qualify for the new standards.

On many places it’s already illegal to modify or tamper DRM protected content in any ways (does anyone remember libdvdcss?) and the plan is to include similar (more or less) restrictions to the whole world wide web, which would say that we’ll have things like fediverse who allow browsers like firefox and ‘the rest’ like banking, flight/ticket/hotel/whatever booking sites, big news outlets and so on who only allow the ‘secure’ version of the browser. And that of course has very little to do with actual security, they just want control over your device and what content is fed to you, regardless if you like it or not.

Those are made on Firefox engine. That is made and maintained by the company Mozilla. Which is experiencing those problems.
It’s like those people who say that they don’t use chrome because it’s shit and breaks privacy, they use edge and brave.