Firefox on Android is supposed support MSE (for MP4/H.264, but not WebM/VP9). For example, YouTube MSE works in Firefox. Is it broken for you? That JW Player page is out of date. It also says MSE doesn't work on Firefox desktop. :)
Yes. There was a temporary whitelist while Firefox MSE was still in development (to workaround some API incompatibilities on some video sites), but the MSE whitelist was removed in Firefox 42 (November 2015) allow all websites to access the MSE APIs:
MSE has been available on Android since Firefox 41 (bug 1084456). That MDN page was out of date. Thanks for pointing out the error! I just updated it. :)
Firefox Android doesn't have Media source Extensions, that allow HLS support in HTML5 : http://www.jwplayer.com/html5/mediasource/