Add support for firefox 28.

data/js/detect/os.js

@@ -212,7 +212,9 @@ window.os_detect.getVersion = function(){
 		// Thanks to developer.mozilla.org "Firefox for developers" series for most
 		// of these.
 		// Release changelogs: http://www.mozilla.org/en-US/firefox/releases/
-		if (css_is_valid('cursor', 'cursor', 'grab')) {
+		if (css_is_valid('flex-wrap', 'flexWrap', 'nowrap')) {
+			ua_version = '28.0';
+		} else if (css_is_valid('cursor', 'cursor', 'grab')) {
 			ua_version = '27.0';
 		} else if (css_is_valid('image-orientation',
 		                 'imageOrientation',