From MozillaZine Knowledge Base

Currently there is no Show Image context menu item in Firefox.[1]

To have this feature added to the browser install one of the following extensions:[2]

• The ImgLikeOpera extension
• The Adblock Plus extension
• The Show Image extension

There is a pref on the about:config page to add a 'Reload' item to the context menu: extensions.showimage.display_reload_image_context_menu [3].