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.
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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.