My problem is that when I place :focus style for our dropdown lists like so:

input, select {
    margin: 0;
    padding: 0;
}

select:focus  { background: #FFC; }

...the dropdown with the focused selection suddenly has an extra spacing on all sides (1-2px spacing). See images below:

It may seem unnoticable on the photos above but when you do a selection, the increase in padding/spacing cannot be missed.

I already tried removing margin: 0, padding: 0, thinking that the :focus state might be reverting to the padded version but still the same. Has anybody encountered this problem?

Only happens in Firefox.