Like the argument with copyright, it's not theft. It's only theft if the user is deprived of something.
> In both cases you are taking private data without consent and using it to get money, or money-equivalent things (like a better product or more users)
This is just word salad. Opting in is consent. You are absolutely free to opt out, by following the clear instructions on the website [0], and you can continue to use the software. Your rights are being deprived here.
Secondly, "money equivalent things" is a reductionist argument. Not everything fits neatly into a black and white "right" and "wrong" bucket, and arguing it does is forgetting about the fact that technology doesn't exist in a vacuum any more than any other product or industry.
> Technically and philosophically there is absolutely no distinguishing feature between opt-out "telemetry" and spyware.
Technically there's no difference between me sending any kind of data to a remote service. Using rote logic to infer behaviour is a fallacy, and when I read comments like this I end up (unfortunately) coming away thinking "this person has no critical thinking skills". I'm aware it's an ad-hominem, but logically there's no other explanation other than you can't possibly understand anything other than your narrow world view.
[0] https://support.mozilla.org/en-US/kb/telemetry-clientid