For Firefox, I think that disabling the telemetry and the studies is not going to help Mozilla improve the browser.
And not giving IKEA access to cameras in your home won't let them improve the furniture.
Every single thing for the past 10 years has had (opt-out, which most people didn't) telemetry and that correlates with a decline in quality, not improvement.
One could hope if it happens enough they'd be jostled out of the McNamarra fallacy tarpit they've ended up in, though maybe that is too optimistic.
They don't care about improving the browser anymore. They just want to make it into an AI browser.
I still use Firefox but I, frankly, feel it's stagnated. On mobile I'm in the process of changing habits to something else (auto reflex sometimes still opens Ffox, but lately I'm circling back to opera, which I stopped using on desktop what... 20 years ago?)
All this to say, I don't think Mozilla is doing much with all the telemetry data it's gathered all these years
I'm not convinced that's their goal any more. The number of users turning off or ignoring AI features is probably considered a problem, not a signal.