On Linux it may be that the various distributions decide to repackage Firefox with the default setting flipped. Not sure about the various policies on that one.

The ESR track presumably will have the default flipped because corporations get funny about data transfers to remote servers - mind you Microsoft seem to be getting away with it for business who don't have a full on Enterprise set up.

The way I see it is that if Firefox's userbase dwindles because of this, either we get our Firefox with opt-out telemetry or... Firefox dies. And now we have a Chrome monopoly.

I'm not sure I like that gamble.