Firebug is a Firefox add-on which integrates a host of essential web development tools into the browser.
Firebug's CSS tools make it easy to see which rules are styling individual page elements. Or you can view entire style sheets, if you prefer. If there are problems then you're able to disable any CSS property in a click, just to see how a page appears without them. And not only does Firebug allow you to edit CSS properties, it also knows which keywords you can use, so you don't have to remember every option yourself: just press the Up and Down arrows and the program will cycle through all the possibilities.
Version 2.0.16 (Changelog)