improve transitions for hovercat drop-down menu at desktop width (bug 1096654)

https://bugzilla.mozilla.org/show_bug.cgi?id=1096654

![screencast](https://cloud.githubusercontent.com/assets/203725/4986541/3891af82-693c-11e4-86f5-56eeb5aa06b7.gif)

it's not BEM-compliant or compliant with [anything else](http://clubmate.fi/oocss-acss-bem-smacss-what-are-they-what-should-i-use/). I can just make it one dash. or I could just remove the class and revert to using `.tab-categories`, assuming we aren't going to make this reusable elsewhere. thoughts?