Take a look at the screen shots below.
[Click on the image for all sizes option]

Observe the listings below the search box, viz Top 100, Comedy, Movies.. etc. Right next to the category name is a small downward arrow, a drop down list actually. Mouse-over on any name and a list appears.
On FF (below) the drop down doesn't show up. Why this 'differential treatment'?

P.S. Hope this not an issue with FF Javascript engine.