modify country selector so scrollbar is always visible on firefox (and chrome, etc).

per asingh:
"the system scrollbar is not customisable in firefox; this happens because firefox does not give access to its scrollbar; https://stackoverflow.com/a/18318273/1930053 here is one link for reference. The way to do would be to use a custom slider and not using the system slider. One benefit we would get after this is we'll have same design of the slider across all browsers and OS"

cc @nikos @Konstantinos.Kokkorogiannis