Add transitions on FF and fix .menu-link getting hidden on mobile.
Add transitions on FF and fix .menu-link getting hidden on mobile.

- Fixes #71
- Fixes #66

chensy0203/pure-site,yahoo/pure-site,pandoraui/pure-site,yahoo/pure-site,jamesalley/pure-site,jamesalley/pure-site,weigang992003/pure-css-chinese,weigang992003/pure-css-chinese