/* highlight HTTPS URLs (works on Mac too, if a bit messier there) */
#urlbar[level] .autocomplete-textbox-container > * {
background-color: #FFFFB7 !important;
}

/* Thanks, Ace_NoOne
http://lifehacker.com/396582/turn-firefox-3s-location-bar-yellow-at-https-urls#c6299408 */