Social Fixer is great - I've been using it for a while and it makes my infrequent forays onto Facebook less miserable. From what I can tell, Facebook has removed the Social Fixer page on Facebook and are requiring the author to make significant changes in order to return it. The author is going to make those changes.
Why not just lose the page on Facebook and keep developing Social Fixer? If they threaten legal action, throw the code on Github and stop development on it under your own name. Or put it in an LLC or equivalent and just liquidate if they sue. Why does capitulation seem to be the most appealing option?