`dom.private-attribution.submission.enabled` already was false. If you're having to set it manually in `about:config`, you're likely missing something bigger in `about:preferences#privacy`.
> Allow websites to perform privacy-preserving ad measurement
Strangely, though, it was already unchecked, and at any rate, greyed out so I couldn't toggle it. Toggling that about:config value didn't change the state of that checkbox.
I also found another about:config pref called "dom.origin-trials.private-attribution.state" (set to zero for me); I wonder if that has anything to do with it.