This add-on (and its siblings from the same developer) behaves nicely. It alerts you to fingerprinting attempts, appears to successful obfuscate your fingerprint, based on a number of testing sites I have tried. It asks for minimal permissions and you can toggle the alerts from the toolbar icon. The website it links to explains the functionality clearly.

The only thing missing is that it is not a Recommended Extension and/or endorsed by an objective organisation, ideally the Electronic Frontier Foundation (EFF).

I also wonder how necessary this is. Firefox claims to resist fingerprinting nowadays (https://blog.mozilla.org/firefox/how-to-block-fingerprinting-with-firefox/), but my assumption is that this is a more paranoid effort, which suits me. I would be interested to know more.