Works well on MacOS when others don't. Many thanks to the developer.

Works well in Firefox Quantum on macOS High Sierra. To ensure the dictionary was set up correctly, I entered "dict://test" into the location bar and associated the native macOS dictionary with "dict//" links. After that, all good.

Thanks for the great extension!

Works as advertised! It is now possible to lookup words in the macOS dictionary app. I'll second the surprise that this extension is necessary. In Safari and Chrome it is a native feature.

Works as promised :) Such a weird thing that Firefox actually needs this extension, though.