[horde] Firefox missing plugins

[Louis-Philippe Allard]

Every time I access my calendar, Firefox complains of a missing plugin.  I
see a warning bar at the top of the screen saying:

  "Additional plugins are required to display all the media on this page" 
but when I click the "Install Missing Plugins" button on the right, the
Plugin Finder Service opens up and says : "No suitable plugins were found".

  Which plugins are missing in my installation that Horde uses ?