Firebug is a tool which will help you to develop, alter and improve your Wikidot site a it shows you the CSS (Custom Style Sheet code) that is being used for everything on your page. It enables you to test changes on the fly and fix any problem with your CSS. For many Wikidot gurus Firebug is an essential part of their web development toolbox. For people new to CSS it is a great way to see what rules are being used and to learn CSS.
Firebug is for Firefox. There is a lite version for IE and Chrome but it has significantly less functionality. If you use Safari 5 on a PC or Mac there is a Web Inspector extension which is similar to Firefox.
The Firebug website is at http://getfirebug.com/