It's an XUL page- the XML-based files Firefox (and other Mozilla applications) use to define their interfaces. It'll resolve perfectly well if opened in Firefox, but other browsers will have no idea what to do with it.
Having just used it to choose maneuvers for a character that comes in with his first Swordsage level at Initiator Level 4, I must say that it is awesome. Its possibility of sorting on any coloumn, and filtering out everything that isn't for a specific class, for a specific level, etc., combined with its listing of the number of prerequisite maneuvers each maneuver requires? Makes things much, easier.
Look at it this way: Firefox is a utility to make maneuver-choosing easier - it's worth installing just to read this file.