Show the settings gear in reviewer tools tablet width (bug 980349)

https://bugzilla.mozilla.org/show_bug.cgi?id=980349

Before
![screenshot 2014-07-08 13 36 05](https://cloud.githubusercontent.com/assets/211578/3513819/7334d408-06c6-11e4-83ca-7bfe487695a6.png)

After
![screenshot 2014-07-08 13 35 48](https://cloud.githubusercontent.com/assets/211578/3513825/774133ac-06c6-11e4-930d-2fa39000d443.png)

keep in mind that Firefox 29 and lower require `-moz-box-sizing: border-box;`