It seems that rule is not taking effect. Do you have CDN caching the css or something?
Can you try to replace it directly to see if it works inside the main.css
replace this
.browser-ff select, .browser-ie select, .browser-ie10 select {
background-image: none;
}
by this
.browser-ff select {
background-image: linear-gradient;
}
.browser-ie select, .browser-ie10 select {
background-image: none;
}
-Rui