From MozillaZine Knowledge Base
Currently there is no Show Image context menu item in Firefox.
To have this feature added to the browser install one of the following extensions:
- The ImgLikeOpera extension
- The Adblock Plus extension
- The Show Image extension, which includes a preference that adds a 'Reload' item to the context menu, when set to true in about:config:  extensions.showimage.display_reload_image_context_menu