fix that ugly border on pretty-selects (bug 1093936)

related: https://bugzilla.mozilla.org/show_bug.cgi?id=1093936

# before & after, respectively
![screenshot 2014-11-19 17 52 18](https://cloud.githubusercontent.com/assets/203725/5136505/0c46e34e-70de-11e4-897b-6898571b9bed.png)