Make navigation look a bit nicer in firefox

My miniscule contribution...

* Made scrollbar automatic instead of always showing
* Added a little bit of margin to the right of the level 2 style headings

Screenshots:

Before, you can see the scrollbar is slammed right up against the text.

![beforewwewew](https://cloud.githubusercontent.com/assets/2295892/2628109/2333c9c8-be1e-11e3-8533-80acbd94afc5.png)

After, it only shows if it's necessary. When it does show the text has a margin so it still looks SWELL

![screenshot from 2014-04-07 16 29 01](https://cloud.githubusercontent.com/assets/2295892/2628116/5f19db8a-be1e-11e3-820c-653f4298bdc4.png)

![screenshot from 2014-04-07 16 29 15](https://cloud.githubusercontent.com/assets/2295892/2628117/627bf0d8-be1e-11e3-9a84-a4932260e97c.png)