Select arrow Firefox 30

Refers to #5312.

 - Removed `url-prefix` media query for styling `<select>` elements;
 - Replaces the base64 `background-image` for a version with horizontal spacing;
 - Tweaks the `background-position` property so the arrow is always fixed 100% on the edge (no 97% anymore);

Works everywhere, and on Firefox the custom arrow is now hidden under the native one.

Firefox:
![screenshot 2014-06-11 18 59 12](https://cloud.githubusercontent.com/assets/766837/3246202/36416474-f179-11e3-9fd5-bb45820104ec.png)

Chrome:
![screenshot 2014-06-11 18 58 49](https://cloud.githubusercontent.com/assets/766837/3246206/432f60dc-f179-11e3-877e-359241700760.png)