It would be like using Tor all the time. You know it's better, but the difference in speed is so great, that you have to make a conscious choice. I think I speak for a lot of people when I say that I want the benefits _without_ compromising speed.
Also, Google's pushing it helped. Otherwise consider how Chrome has occupied an IE like market share despite the quality Delta between Chrome and IE/Edge, Firefox or Safari is significantly lesser than the Delta between Firefox and IE was a decade ago, yet Firefox peaked at a much lower marketshare than Chrome's today.
Firefox has a chance at doing the same thing with privacy-aware users and/or power users.
To that end, they need to stop dumbing-down the browser and endlessly playing catch-up with Chrome.
Firefox is looking like Apple circa 1997. They can still recover, but they have to focus on what people want.