Comment by jjnoakes

A/B testing implies extra tracking and reporting of behavior, to distinguish between the A group and the B group's response to the change.

On a website, that isn't too much of a problem, because you expect your activity to be tracked since you are interacting with a web server.

In a browser, the same tracking is more intrusive. My behavior on a site is now being sent to the browser author, an unrelated party.