We blogged a lot about performance problems in Internet Explorer caused by the missing native implementation of getElementsByClassName (101 on jQuery Selector Performance, 101 on Prototype CSS Selectors, Top 10 Client Side Performance Problems, …). Firefox on the other side has always implemented this and other native lookup methods. This results in much faster page load times on many pages that rely on lookups by class name in their onLoad JavaScript handlers. But – this is only true if the web page also takes advantage of these native implementations.

Yahoo News with 1 second CSS Class Name lookups per page

Looking at a site like Yahoo News shows that this is not necessarily the case. The following screenshot shows the JavaScript/Ajax Hotspot analysis from dynaTrace AJAX Edition for http://news.yahoo.com on Firefox 3.6

... read the rest of the article that explains how Yahoo News spends about 1 second in old framework code