If this thing needs to be present, the option should be there to toggle on, not off.
This is my takeaway in general. The idea of this sounds fine, but the fact that they opted everyone into this experiment is really stupid considering a huge chunk of people use Firefox are privacy-conscious and care deeply about this stuff.
Not all cookies are harmful and some websites don’t work properly without cookies. Having cookies off by default also usually means user preferences wouldn’t be saved when you leave and return to a website.