Unfortunately, "Look up dictionary by using Right Click" is not very useful on my Mac. The first word I select for lookup works fine. However, after that, the contextual menu item disappears, and the only way to look up another word is to reload the page, at which point the contextual menu item reappears for one more lookup.

BTW, I tried the "dict://test" suggestion of another reviewer (which I was not familiar with), and that test works fine on my Mac. I have no idea why this extension only works for a single lookup, but that makes it non-useful in my opinion.