Bug 947364: Adds overflow ellipsis for filter drop-down

([Bug 947364](https://bugzilla.mozilla.org/show_bug.cgi?id=947364))

Before:
![Filter view: before](https://dl.dropboxusercontent.com/spa/49xeskca52abzlc/b44e0nwk.png)

After:
![Filter view: after](http://snaps.akibraun.com/ytst7.png)


I'm not totally certain this is the ideal solution, but it's definitely an easy-to-do solution for right now.