It used to support it out-of-the-box as well, but it's technically against YouTube's ToS to allow this without paying for a premium, so now you need this as an extra hoop.
Agreed, this sounds strange indeed. Much more likely is that Google found a reliable way to detect the screen status using a standard feature and Mozilla just implements the standard neutrally
Wouldn't know, as I have never been in charge of one, but I imagine Google having the power to make your browser completely irrelevant would be a pretty strong incentive.
Nah I want general media playback in the background. Doesn't matter if its Youtube or any other platform. I dont want giga corpos to monetize my attention. Youtube does well enough from ads anyway ;)