Select element background missing on Firefox

Hi,
I noticed that <select> element background (i.e. up/down arrows) is missing in Firefox due to this lines in Enfold CSS:
.avia-mozilla #top select, .avia-mozilla #top .avia_ajax_form .select, .avia-mozilla #top .entry-content-wrapper select {
background-image: none;
}

I tried this on Firefox 63.0.3 (64 bit), and you can check it out here:

I’m sure there is a good reason for that, but I can’t fiure it out!
Maybe it’s because older version of Firefox had their own arrows for <select> element.

Anyway maybe those line in Enfold’s CSS are not necessary ;)

Alessio