A growing number of instances (mainly of Mastodon so far) are signing an ‘Anti-Meta Fedi Pact’, pledging to block any instance owned by Meta in the fediverse.
I don’t know how big this will get or how effective it will be, but if you run a fediverse instance, you should take a look at this https://fedipact.online/
I agree with your point but I am curious about the sites blocking Firefox part. I have been only using Firefox now for many years and apart from one or two poorly built government websites, have never run into a site that didn’t work.