Hmm, so what is the status of this W3C draft[1] (which they say specifically does not imply endorsement), if the W3C is putting a test for it in one of their test suites? Or do Chrome/Chrome devs get commit bits? Or maybe one of the purposes of this is to serve as a way for browsers to coordinate on implementations even before it's become a standard? (It's obviously a way to coordinate on things that are a standard).
One of the things it merges is BZ # 1442425 which is landing a newer version of the W3C's set of web platform tests, seen here testing the Typed OM feature you are interested in:
https://github.com/w3c/web-platform-tests/tree/master/css/cs...
So, this actually tells you nothing about whether Firefox is interested in implementing this or plans to do so.