Well, it's strange, it sometimes works and sometimes doesn't.

However, apparently, your web server returns .js files with the "text/html" content-type, which isn't good. Quite probably, Firefox sees that and doesn't interpret the script, which results in "DynarchMenu is not defined" in JavaScript console.

Please correct your server installation--make it return appropriate content-type headers for JavaScript files ("text/javascript").

PS: your skin looks cool. :-)