This tutorial code works fine in Safari 3.1.1 and Opera 9.5.0 but is failing in FireFox 3.0. These are all Mac OS X versions. In the case of FF3 when you click the link jquery.com is loaded. The code which adds the test class to the a element does work in FF3.
<html> <head> <script type="text/javascript" src="jquery.js"></script> <script type="text/javascript"> $(function(){ $("a").click(function(){ event.preventDefault(); $(this).hide("slow"); }); $("a").addClass("test"); }); </script> <style type="text/css">a.test { font-weight: bold; }></style> </head> <body> <a href="http://jquery.com/">jQuery</a> </body> </html>