That's a cat-and-mouse game, and the Firefox release cycle is bound to be much slower than any website interested in tracking users. The owners of the site can tweak the JS slightly, the FF devs have to get notified that the website stop working, go back to see what changed, and stage the necessary changes for the next update. And then wait for users to update, of course.
It's not an arms race for the sites that aren't purposefully breaking. Shims would fix accidental breakage, and reduce "mysteriously broken" sites. Intentional breakage, at least in my limited experience, is accompanied by some warning text about adblockers.
It's very impractical.