This is probably the best highlighting tool available for Firefox at the moment.

But the blacklist feature ("Never highlight on these sites") is not working. When I add any site to the list it stops the add-on of working on any site and blocks the access to the configuration. To fix it you have to purge the configuration (reinstalling).

Firefox 69.0.3 (64-bit) on Linux

I could not find the repo to try and create a PR.

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Update: if somebody else is experiencing this issue; restarting the browser fixes the problem.