Apparently 4 weeks ago they announced it (https://support.mozilla.org/en-US/kb/privacy-preserving-attribution).
I disabled the setting.
This is point 1 of 4. It just gets weirder after this.
"Websites that show you ads can ask Firefox to remember these ads. When this happens, Firefox stores an “impression” which contains a little bit of information about the ad, including a destination website."
"If you visit the destination website and do something that the website considers to be important enough to count (a “conversion”), that website can ask Firefox to generate a report. The destination website specifies what ads it is interested in. "
there's more. what a tangled web we weave. this has been a PSA.