I still think it was a mistake for Firefox to dump its old plugin model. The customisation was a USP for Firefox and many useful tweaks and minor features have never been replaced.

Today the ability to run proper content blockers is still a selling point for Firefox but obviously wouldn't be if they started to meddle with that as well. (Has there ever been a more obvious case of anticompetitive behaviour than the biggest browser nerfing ad blocking because it's owned by one of the biggest ad companies?)

Other than customisation the only real advantage I see for Firefox today is the privacy angle. But again that would obviously be compromised if they started breaking tools like content blockers that help to provide that protection.