I love firebug, this is something really good for developing web apps. But today, I got an issue which was pretty annoying! First of all, when I develop a small apps, I used to do this under firefox only with firebug and other nice extension loaded.
But today I got an issue when I wanted to overload the XMLHttpRequest send function to do other things with: Firebug simply do not allow me to do this, but it works well if I want to overload the 'open' function!

Pretty annoying but you cannot do this with firebug activated:

XMLHttpRequest.prototype.send = function(data) {
    sData = transformation(data);
    this.originalSend(sData);
}