Edit: this trick stopped working as of Firefox 30 realeased in 2014-06-10. Join the comments section to help finding an alternative, and please upvote the bug on Bugzilla for a definitive fix. If you now have double arrows on Firefox, this might solve your issue.
remove hide the select arrow in Firefox using
TL;DR (or, the fix)
none. This will "reset" the styling of the element;
0.01px. This will "push" the text a tiny bit to the right;
''(an empty string). This will change anything that extends beyond the element's width to... nothing - *