For me, it also doesn't work. macOS Ventura 13.4.1

not working anymore

used to work, until Big Sur.
Now, sadly, nothing happens. Version 1.3.3

debuggee 'resource://devtools/shared/base-loader.js:289' would run builtin-modules.js:196:11
window.gBrowser is undefined ContentDispatchChooser.jsm:71
handleURI Choo
AsyncFunctionThrow self-hosted:678

Giving it 3 stars because it used to work, but for me has stopped (Mac High Sierra).

I can confirm that this does not seem to be working on either macOS 10.15.5 or macOS 10.12.6. Is this still being supported by the developer?

Like Firefox user 15573474, I'll update my rating to five stars once the add-on is working again.

UPDATE: Hi Emmanuel, thanks for your reply. I've just updated my macOS to 10.15.6; I don't know if it's something in my settings, but the dictionary is still not opening for me when I select a word, right click, and choose "Look up "[selected word]" in Dictionary." Nothing seems to happen at all, in fact. Please keep me updated if there are any further developments. I'll remove the two-star rating, since I know you're working on it.