Hide the select arrow on Firefox

Refers #5115 and #4236.

Hides the native arrow on `<select>` elements on Firefox. This workaround *might* stop working when Firefox 31 is released, but even if this happens, the native arrow will cover up the background one.

It's worth mentioning that the guys at Mozilla are [now proposing](https://bugzilla.mozilla.org/show_bug.cgi?id=649849#c111) a definitive fix. I wrote [this gist](https://gist.github.com/joaocunha/6273016) which gives the full lowdown on the subject. It's a safe workaround to use, nevertheless.

mdo is using the workaround [on his wtf-forms](https://github.com/mdo/wtf-forms/blob/master/wtf-forms.css#L166) as well.

Before:
![screenshot 2014-06-08 18 46 55](https://cloud.githubusercontent.com/assets/766837/3211553/77c75f18-ef1f-11e3-948c-879310507f29.png)

After:
![screenshot 2014-06-08 18 46 16](https://cloud.githubusercontent.com/assets/766837/3211556/7e9b4962-ef1f-11e3-96e6-71ec9d7f3e1e.png)