It may be a nice feature, but for me Firefox's implementation seems to be a bit too intrusive, because as a developer I can't disable it and it is always visible as a browser-styled overlay.

Not all video elements are for stand-alone consumption. For example, there are "hero" videos in landing pages, enterprise solutions requiring smart overlays, or imagine a component in an online video editor.

Firefox also have no plans to support the Picutre-in-picture web API: https://www.w3.org/TR/picture-in-picture/.

See, for example, this bugzilla issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1611831