I have been looking for this for so long.

Firefox 105.0.1 and macOS Monterey 12.6: plugin working like a charm. I've been stuck with Firefox to work (I rather use Safari) and I was *badly* missing 'Lookup'. Thank you, Emmanuel!

I works fantastically in my Big Sur macbook. What you have to do is to allow firefox to always run the MacOS Dictionary by typing "dict://test". in the address field of your FF browser and click "always..." and now it will always work.

Works on big sur (11.2.3) with Firefox 87.0. Simple yet effective, thanks for sharing this!

This extension adds a "Lookup in dictionary" option when you right click a selected word on a page. What this does is open the as "dict://". You can try the same thing from the browser bar. E.g. type "dict://test". I noticed that I first had to use the "dict://" URI from the browser bar manually and associate it with the OSX Dictionary application before the plugin would actually do something for me.

works as expected on Big Sur 11.2.3 and Firefox 87

revised 27/09/2024: still working at macOS12.7.6 w. FF127.0.1, if you've issue "Always allow ..." go FF about:config>security.external_protocol_requires_permission cf.: support.mozilla.org/en-US/questions/1321723
---
Suddenly not working at FF75.0 @macOS10.15.1, pls. help. how to debug? thx!
EDITed after 9 months, still not working at macOS 10.15.6. why? how to debug? pls help!
[2021.02.10] Would still appreciate if you could update this, because it is not working. why?

Extremely small app which requires no dodgy permissions as it uses the mac's built-in dictionary. Nice.

UPDATE: I can't get this to work on MacOS 10.15.7 either, for both Firefox 84 Beta and Firefox 83. Tried granting Firefox.app Full Disk Access in System Preferences -> Privacy, to no avail.

Uses the in-built MacOS Dictionary.app rather than leaking your dictionary requests to online services. Works as advertised.