Many people who do web design would agree that the Web Developer Extension for Firefox is one of those must have tools that you can't live without. Now there is another to add to your toolkit: Firebug.

Take a look at this screenshot. Notice how I've highlighted some HTML code on the left. Then Firebug displays the CSS which applies to this code on the right and, as you can see, even from multiple style sheets (like Knews uses). I also like how it displays the enclosing block elements above beside where it says "Inspect" and "Edit."

There are other features I haven't experimented with, but I'm sold. This is not a replacement for the Web Developer Extension, but definitely a must have addition to it if you are doing CSS design or studying the CSS on other websites.