Here are a few of my favourite addons
Similar to firebug, better in some ways worse in others. Whichever is better is a matter of opinion.
Has various colour tools but the main strength is that it gives you an eyedropper that can grab the colour of anything on the screen. It then lets you copy that colour to your clipboard in a variety of formats, hex, rgb etc.
Allows you to quickly see how the current web page would look like rendered using Internet Explorer