Firefox has several specific contexts in which different right-click menus are displayed. I (and I think most other users) would vastly prefer the menu item for "invert image" to only show up in the "image" context, as this would save confusion and many accidental clicks.
The add-on show up in the "any" context to invert any part of a page for example: select parent element to invert group of an images; same to invert pdf on github..../google docs/video element/body of a page.
Firefox has several specific contexts in which different right-click menus are displayed. I (and I think most other users) would vastly prefer the menu item for "invert image" to only show up in the "image" context, as this would save confusion and many accidental clicks.