[0] https://hacks.mozilla.org/2018/10/testing-privacy-preserving...
It'd be a lot more acceptable if there was an option to show me "This is the exact telemetry payload we want to send to Mozilla." And even then you are taking someone's word for it that there isn't some other piece of data hidden in a hash or something, or that the browser isn't secretly sending data.
I'm not quite paranoid enough to do the full monitoring of all network traffic, but how do I reasonably know what is going on without listening to traffic/watching memory at all times? In the end, I'm trusting a faceless corporation that is attempting to put on a facade of trustworthiness.
The only trustworthy computer is an unnetworked one.
Eventually, you have to just get on with your life and trust people.
There is! Navigage to about:telemetry in Firefox.
Actually you aren't. It's quite easy to build your own Firefox and use that, or use say Ubuntu's build if you trust them more. You wouldn't want to audit the Firefox source code yourself, but if Mozilla was intentionally sneaking backdoors into the source code, someone could (and probably would) eventually find that out and you'd be able to verify it by examining your source archives... which means Mozilla would have to be stupid/crazy to try that.
At the end of the day if you want Firefox to make rational decisions about what feature to implement they need to collect some data. That fact alone can't be a problem for anyone who wants to have firefox be a competitive browser.