Thanks for the feedback!

If you're referring to cases such as YouTube videos where a custom context menu replaces the original one, you can use Ctrl+Right Click or double right-click to see the original context menu in Firefox.

If the original context menu also does not show the button, then either there are no images under the cursor (e.g. some pages uses special fonts to print icon images, in which case behind! cannot open the font character as an image), or the add-on is unable to access those images (e.g. due to browser security precautions, behind! cannot work on iframe tags).