Good add-on ! I was expecting that the dictionary pop-up will open (like in Safari), and not the full dictionary app.

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
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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.

So far, so good. The Add-On is unobtrusive and does what it claims to.

I'll revisit in a few weeks and bump it to five-stars assuming everything continues to work well.