Firebug is great for CSS because it allows you to find the specific CSS rules for a selected element, along with the line number it appears on in the CSS file(s), helping you find it in the separate text editor in the first place. Also you can try out new rules very quickly and get instant feedback without switching programs and setting up an auto-refresh.

I switched to Webkit's JS debugging tool not long ago, it has much better warnings than Firebug.