It didn't work for me either, but apparently one of the issues is that it assumes window.speechSynthesis is available which may be disabled via about:config > media.webspeech.synth.enabled.

Windows 10 w/Firefox 130.0, ublock, privacy badger, and works ok for me?

I'm not the dev, but figured it might help them if they know more details. You on Linux, Win11, Mac, BSD, Mobile?

Android + Firefox, works ok.

works for me on MacOS 14.6 FF