Fix side menu so that it stays open in Firefox and displays correct icon

While browsing the documentation, I saw that in Firefox it was not possible to access the nested menus in the side menu. This PR modifies the display logic so that 1) Submenus are visable in Firefox and 2) The correct icon is displayed in all browsers.

**When in Firefox:**
![screen shot 2013-09-15 at 3 18 44 pm](https://f.cloud.github.com/assets/307685/1146155/6ea1c772-1e49-11e3-843b-22ba754b13f3.png)

**In all browsers:**
![screen shot 2013-09-15 at 4 57 59 pm](https://f.cloud.github.com/assets/307685/1146160/ae19c396-1e49-11e3-81d3-07882c4f7ea0.png)

If I am missing something in this PR or the logic isn't right, please let me know and I'll be happy to update the PR.

Cheers!