+2  A: 

Try to set the -moz-appearance CSS property to 'none' or 'none !important'

CMS
Unfortunately that didn't work. :(
takteek
What is "that", which didn't work? You probably didn't do it correctly.
Nickolay
+1  A: 

Like the previous solution but only using javascript :

var pnl = document.getElementById("panelId");
pnl.label = "OK";
pnl.style.color = "white";
pnl.style.backgroundColor = "green";
pnl.style.MozAppearance = "none"

Note that after you do this you'll pretty much have to style it from scratch again and it will probably also lose it's OS specific style.

lithorus