X-browser AJAX PHP bug

PHP/MySQL-based site in beta has a user profile page that uses AJAX and PHP to offer a few simple Lightbox forms; load items into database and refresh an on screen list. It works in most browsers (inc. IE, Firefox and Chrome in Windows) but have discovered that it doesn't work in some others (eg. Safari on the PC or Firefox on the Mac). Looking for someone to fix this. Potential for lots more work in future.