Without Firebug, you're left scratching your head when the headlines on your page come out blue, when they should have been black. With Firebug, inspect the problem element and before you can blink you'll see the CSS that is causing the problem.
Firebug shows you the rules that cascade together to style each element. Rules are sorted in the order of precedence, and properties that have been overridden are stricken out. Each rule has a link back to the file where it came from which you can click to jump to the line.