I’m banking (ha) that most web dev is lazy and won’t change shit that isn’t broken. It’ll be YouTube mainly since Google hasn’t figured out how to stop uBlockO.
Most other websites are probably not worth it and the Internet is designed day one to route around damage. A whole bunch of Blogspam SEO sites banning Firefox is a win.
Otherwise they’re be a addon extensions for Firefox developed in a week probably to “fake” it.