Web Developers shouldn't need to "support" particular browsers at all, that's what standards are for. Nobody should be "supporting" any non-standard functionality at all.
I agree. But different standards get adopted by different browsers at different rates. Case in point: Firefox still has not released support for declarative shadow DOM.
As a side note, although related to standards: I thought Firefox insisted several years ago that they were not going to support File System Access API. But now MDN lists it among supporting browsers.