See web-platform-tests/wpt#9339 (comment).

The spec says:

The values (including the defaults) in ellipse() and circle() serialize to their 2- and 4-value forms only, preferring the 2-value form when it can be expressed without calc(), preferring left and top origins, and preferring 0% over a zero length.

Firefox right now special-cases that to follow the spec, but I think it'd be more sane and consistent to just use the regular serialization. Is there any reason that sentence was added to the spec?