If you want to disable this, instructions are given here[1] but 1) Hamburger menu -> Settings -> Privacy & Security 2) scroll down to the new section entitled "Web Site Advertising Preferences". 3) Make sure the box marked "Allow web sites to perform privacy-preserving ad measurement" is not checked. [1] https://support.mozilla.org/en-US/kb/privacy-preserving-attr...
The underlying pref is dom.private-attribution.submission.enabled. I'm going to force this off in my policies.