Fixes firefox image hidden overflow bug.

There is currently a small bug with the Zoom Image feature in firefox. This fixes it. 

Before (image gets cut off at bottom when in fill screen modal mode):
![screen shot 2014-05-01 at 11 35 08 am](https://cloud.githubusercontent.com/assets/3364609/2855664/74bd0cec-d15f-11e3-90c4-f7f324189a71.png)

After (fixed):
![screen shot 2014-05-01 at 11 35 33 am](https://cloud.githubusercontent.com/assets/3364609/2855667/7e4157e6-d15f-11e3-9416-f6f988d15ea9.png)