I don't see the Zarafa right mouse menus, but only the default Firefox windows. What's the reason for this?

This happening because the option ‘Allow script to: Disable or replace context menus’ is disabled in FireFox. To enable this option go to:

Tools > Options > Content 

Make sure that 'Enable JavaScript' and click on the button ‘Advanced’ behind ‘Enable JavaScript’. Here is it possible to enable the option ‘Disable or replace context menus’.