Update 11/5/2008: There’s now an extension for this!
Of all the context menus in Firefox, my second most-clicked item (behind View Page Source) is View Background Image. This stock functionality is one of the many reason I still prefer to use Firefox over Safari, even though Safari’s rendering is notably faster. The problem I have with View Background Image though is that the image I want to view is often associated with a link element. When you right-click on a link though, this menu item is sadly missing.