I am a bit of a control freak when it comes to visual formatting, whether it’s figures in a paper or this blog. If you have a web page and are trying to edit its formatting, it can quickly become a real pain to find out which bits of CSS you have to edit to get the desired visual effect, especially if you didn’t write the code in the first place.
Firebug is a Firefox extension that allows you to inspect the HTML of elements on a web page and edit its CSS live. If that sounds magic, well, it kind of is. So if you have an element that’s off by a pixel or two (damn you Vernier acuity!), you can simply click on it, edit its CSS, and immediately check if it screws up anything else. Awesome.