How to easily find out what is wrong with a failing Ajax request (using Firefox's Firebug)
When working with ajax, you have most certainly encountered the dreadful "Error: AjaxHandler.actionRequest(): received response is malformed!" error message. In development mode you can display the real error message but it is not always convenient and in production mode you can not see the real error message at all.
So here is an easy way to always and easily be able to see what is the real error message:
Install firebug in firefox.
Access your project in Firefox.
On the lower right of the Firefox windows, there is now either a green checked circle or a grey crossed circle. If it is grey, clic on it, you should see "Firebug is disabled", clic on "Enable Firebug".
In firebug, click on the "Net" tab and verify in the options (on the far right of the window) that Disable network monitoring is NOT checked. Uncheck it is so.
Now select the "Console" tab and, in your project, do any action. You should see in the "Console" a new line with "POST htpp:// ..."
If you clic on the [+] you can see 4 tabs: Params, Header, Post and Response.
What interest us is the Response tab.
If you do an ajax request and you get a Error: AjaxHandler.actionRequest(): received response is malformed!, simply look in the "Response" tab to see what was the raw message returned, it will contains the exact error message that will tell you what was wrong.