Firefox 4 which was released yesterday has several new and interesting features for developers. One of the features I found interesting was the "Heads Up Display" Web console feature which allows you to debug any website and see errors, info and warnings along with network connections being made by a website.
To enable the Heads Up Display web console debugger, just press Alt + T and select Heads up Display or you can also use the Ctrl + Shift + K hotkey to activate it.
This is definitely a very interesting features and I would like it to be more advanced in the future versions. Being a developer I use tools like Tamper Data and Firebug to help me out with debugging websites and requests they make. An inbuilt debugger is just an icing on the cake.