style.css - Original File
.box { -webkit-border-radius:8px; -moz-border-radius:8px; padding:10px; }
style.cs
Firebug handles only Gecko's native rules (styles). Gecko is a browser engine, used in Firefox (http://ru.wikipedia.org/wiki/Gecko).
-webkit is used by another browser engine, so it is ignored.
similar to -ms ("Trident" engine - IE8), -o (Presto - Opera), or -khtml (KHTML - Konqueror).
I don't think you can - the correct behaviour for browsers is to ignore style rules they don't understand. Firefox sees -webkit-xxx and effectively removes it from the style set, so it can't ever be applied to .box.
Safari's inspector will show the -webkit- rules but ignore the -moz- rules for the same reason.