These docs are a general web developer resource that Mozilla maintains - they are not specific to Mozilla products.

MDN is meant to be a somewhat universal resource for web development. I've even seen official Google web dev resources pointing to MDN.

And if you look at the badges in the matrix, the API is not a standard yet, either.

You'll find many features in MDN docs that applies to.

That's mainly because of the expansiveness of MDN docs, but somewhat due to the funding distribution of Mozilla going to officers, not Firefox devs.