Prevent hover effect to be a double underline in Firefox

## Before

![screen shot 2014-02-25 at 21 11 32](https://f.cloud.github.com/assets/85783/2263073/6ea53c84-9e61-11e3-8ffe-60cfc127ac6e.PNG)


## After

![screen shot 2014-02-25 at 21 11 22](https://f.cloud.github.com/assets/85783/2263074/7248281a-9e61-11e3-82a8-324e5e06f2b2.PNG)


![ ](http://s1.developerslife.ru/public/images/gifs/8a0180a4-1ef0-4ac3-b67d-12fa8fb497a7.gif)