In Firebug, how to tell what is overriding a style?

Question

I have this css:

fieldset li {  
padding-bottom: 0em;  
}  

However, it wasn\'t behaving properly, and using firebug, I see that style has

Answer 1

What ever styles are above the crossed out styles are usually overriding it. If that is not the case, start clicking the disable style icons and see where the issue is.