Firefox issue with cancelFullscreen

[swashbuck]

When exiting a fullscreen video with the ESC key in Firefox, an error is thrown:

Uncaught (in promise) TypeError: Not in fullscreen mode
    cancelFullScreen file:///mediaelement/build/mediaelement-and-player.js:6875
    exitFullScreen file:///mediaelement/build/mediaelement-and-player.js:1505
    fullscreenChanged file:///mediaelement/build/mediaelement-and-player.js:1372

I would suggest checking if the browser in fullscreen mode first using isFullScreen():

exports.cancelFullScreen = cancelFullScreen = function cancelFullScreen() {
	if (!isFullScreen()) return;
	if (hasWebkitNativeFullScreen) {
		_document2.default.webkitCancelFullScreen();
	} else if (hasMozNativeFullScreen) {
		_document2.default.mozCancelFullScreen();
	} else if (hasMsNativeFullScreen) {
		_document2.default.msExitFullscreen();
	}
};