I kind of understand why they took this out of firefox; there are different types of “secure” certificates. But this method is a damn site more visible than a blue or green background behind the favicon. (and hey, you still get those…)

I’ve modded it slightly for anyone with a mac, to hide the overflowing background issue you’ll noice if you use the linked version:

#urlbar[level] .autocomplete-textbox-container > * {
background-color: #FFFFB7 !important;
margin: 2px 0;
border-top: 2px solid #ccc;
border-bottom:2px solid #aaa;
}

(it’s still a bit ugly, but better)

(view link)