Javascript context menus

With Javascript it is possible to add a context menu to an object (ie a menu that displays on right-click). However, on Firefox, right-clicking on an object with a custom context menu will show both this menu and the normal Firefox context menu. The Firefox context menu is displayed on top of the custom menu, so in order to access the latter, you need to click somewhere else on the page, in order to access the custom menu.

This behaviour can be overridden by selecting the following options:
Edit → Preferences → Content → Advanced (next to Enable Javascript) → Disable or replace context menus.