Its called ‘stylish’ and essentially overwrites the existing css of the page by using !important. So to remove ads its as easy as finding out the container’s name and setting it to display:none !important;. Easy right? Well actually it can be quite hard to find the names of what you want to change… thats why there is a dev community that has done it for you at http://userstyles.org/

All you do is install the ‘stylish’ add-on for firefox and then search the site for the pages that people have changed. So I searched for gmail and found hundreds of mods for it from simply removing ads to re-colouring the whole site. There are a few nice little applications that are web-wide (like the google ad blocking) and some that are only site wide (I have changed the colour of the Facebook header).

The great thing is, you can write your own or edit existing ones and preview all of them before saving them. Even then its a simple toggle-on, toggle-off system.

I recommend this for every web developer.