I've read this three times now and still don't understand you. Do you think people don't update their software? Are you just counting every update as people happy about the kill switch?
There'll be so much noise in that signal it'll be almost useless. You can't differentiate it from anything else. For all anyone knows it happened because the word Firefox was in the top story on hacker news
Approximately all people update Firefox so you don't need telemetry to count "AI disabled" installations, instead you can derive it as "updates requested" minus "AI enabled".