Nit: the other js files that test "moz" functionality aren't prefixed with 'moz' ("pay" vs. "mozPay", "orientation" vs. "mozLockOrientation"). Perhaps "mozsamplesize" should be "samplesize"?

Nit: "-moz-samplesize" looks a little odd with the '-'. I understand that's the syntax for specifying the image sample size so if you think it's important to be true to the API syntax, that's fine. It looks like other tests are true to the syntax, e.g., mozLockOrientation, navigator.mozPay, so "-moz-samplesize" is consistent.