What it shows is the XMLHttpRequest debugging console, a floating div embedded in the page being debugged. This replaces the javascript console, which the previous script used for tracing.
Each individual XMLHttpRequest invocation will get listed there, with all the details on the request and the response, as well as options to edit and replay the request or replay the response callback.

The user script adds a menu command under “Tools” -> “User Script Commands” -> “Show XMLHttpRequest console". It is mostly useful if you click “hide” in the console menu.