If I were to take a guess (just reading charitably, I haven't actually used Waterfox): they're probably trying to say that the browser doesn't make any requests whose sole purpose is to report your information to them. No telemetry. But browsers do have to send some network requests to their developers for stuff like automatic updates, and they do have to log some of the information received—for example, to tell if someone is trying to hack the update server—but they'll try and keep that to a minimum.

My little snitch popped up like 20 times for callbacks to mozilla and other hosts..

I am thankful for Little Snitch and Open Snitch existence. If those tools didn't exist I will not connect to Interned at all.

Started fighting macOS telemetry and refused transition to Linux partially because of absence of LS alternative. Since Open Snitch I happily run my personal computer with PureOS. https://github.com/evilsocket/opensnitch

The amount of system mothership calling on any platform is mind boggling. And everything has "rational" and "it is inevitable" attached to it. My Firefox looks like Christmas Tree from extensions and my about:config is taking too long to set properly. What a beautiful future we are creating. :)

My guess would be automated crash reports. Those are too good to ignore, and usually don't include much PII.

But its a guess. Would be nice to have devs pitch in here.