Does nothing. Firefox 91.8.0 ESR on Mac OS X 10.14.6.

Works on Catalina (10.15.7) with Firefox Proton versions.

Though not so simple to get going with.

You need to enter "dict://" into the address bar and choose the OSX system dictionary. It will now work for you from now on.

Clearly a bug that needs fixing.

This app doesn't work. Why is it so hard to get Firefox to do a simple function which is define a word?

Doesn't work - simply doesn't do anything at all.

Not working out-of-the-box on Big Sure :-/ Looks neat though!

Doesn't work for me (macOS Mojave).

This doesn't work in macOS Catalina. It worked in Mojave.

Not working on macOS 10.15.5.
I will update the rating to 5 stars, if it is fixed.