There has been an update for the extension which requires you to install it manually for now:
- remove the original add-on from Firefox - download firefox.zip from https://github.com/Anarios/return-youtube-dislike/releases/tag/v3.0.0.11 - go to about:addons - Under the cogwheel - click on 'Debug Add-ons' - click on Load Temporary Add-on... - select firefox.zip
I recommend to check reddit r/youtube where I found this solution.
