If it was originally targetting IE 5.0, then it's somewhat understandable, though lazy. Big stuff like innerHTML and all[] have made their way into the standards and therefore other browsers since so what's left is subtle things like internal layouts of certain DOM objects, various quirks in event handling, unclear points in the standards, that sort of thing (see [link|http://www.quirksmode.org|http://www.quirksmode.org] for more). Depending on how ambitious the JavaScript is and how well constructed it's own framework is, it could only take three or four extra lines to make it all work flawlessly in Mozilla/FireFox.
Can you file a bug? I'd be doing that. There's no real excuse anymore for JS to not be browser-neutral.
Wade.