Statement from the developer:

Distribution of Enhancer stopped for Firefox on AMO, GG Mozilla! Once again, YouTube has introduced new UI changes that break the extension, and after much consideration, I’ve made the difficult decision to stop distributing new versions for Firefox.

Why? The system in place has not allowed me to work at the pace required to keep the extension running smoothly. Not only the developer experience has become awful, but the slow and unpredictable review process for each new version kills all my efforts to fix bugs quickly. So, as long as nothing changes on AMO, there won't be any new version for Firefox...

But, for those of you who are not yet affected by the new UI changes on YouTube, and those of you who never had any issue with the latest version of Enhancer, here is a link to it if you need to reinstall it on Firefox: enhancer_for_youtube-2.0.130.1.xpi For security reasons, I highly recommend you to never install another package! This one has been signed by Mozilla and is 100% secure.

My extension is only for Chrome and Edge now. If you are using Firefox, Brave, Opera, or anything else, do not contact me to report issues as I don't provide support for those browsers...

I used it only for getting sound louder than 100%. BTW new youtube's fullscreen mode is annoying because the only thing that really changed is ability to see some other videos while being in pause mode. Now I download more because this is too much of enshittification for me.