Since Firefox 20.0 on MacOS X popup menus don't pop up any more. With Safari everything works as expected. Tested with Firefox 20.0 with deactivated AddOns.
To reproduce, i.e. go to https://bitbucket.org/fdik/pypeg/ and click on "Followers". A popup menu with the followers should appear, but with Firefox 20.0 it doesn't, at least not on MacOS X.