If you do not like the idea that by default Firefox will send data to Mozilla about your Firefox usage (no matter the privacy protection techniques being used), you should probably be aware that Firefox is already sending this kind of data to Mozilla. This is called Firefox Health Report and is pointed out in their privacy policy: https://www.mozilla.org/en-US/privacy/firefox/#health-report

FHR is also opt-out, i.e., enabled by default. If you do not like this, you may want to disable this as well.