Well, another post on browser behavior. Actually it's about an Add-on. This time it's about a bug I ran into using the Firebug (current version: 1.4.3). Firebug is very popular under Apex Developers, so I thought this might interest you. Firebug actually works fine, but if you are using AJAX calls in your Apex application (or any other web application), you will notice that the AJAX call is made, response received, but not processed. I tried all the other browsers, no problems; just Firefox did not behave correctly. I had no idea. A post on Firefox Support Forum ... another week of cluelessness ... finally I got a friendly hint, directing me to this blog-post: Firefox 3.5/Firebug XMLHttpRequest and readystatechange bug " Looks like this problem is caused by Firebug and solved in the upcoming release 1.5 (currently alpha). Update 5-nov-2009: Firebug Add-in update 1.4.4 is available. I installed the update and the mentioned problem does not occur anymore.