Firefox: Add z-index for sidebar

Adjust other z-index values to 50, and give the dashboard 51

COUCHDB-2234

Screenshot (scroll the sidebar down a little bit to reproduce it): 

![firefox-zindex](https://cloud.githubusercontent.com/assets/298166/4444376/739542dc-47f0-11e4-9eff-a90e79df75d5.png)