Add a max-height to the desktop menu

From #207:

> Also, I tried to give it a max-height and keep the scrollbar intact, but couldn't get it to work

Figured it out.

![firetext-menu-max-height](https://cloud.githubusercontent.com/assets/908303/5233227/1972b35e-7773-11e4-9bfd-88495dd2b0a3.png)