Firefox rolls out Total Cookie Protection by default to all desktop users worldwide | It is Firefox’s strongest privacy protection to date, confining cookies to the site where they were created
Updated Aug. 28, 2024. Take back your privacy Firefox is rolling out Total Cookie Protection by default to more Firefox users worldwide, making Firefox the
making Firefox the most private and secure major browser
If calling home and to selected 3rd party analytics aren't part of the metric then yes, Firefox might be the most private. What proof, even they say they've telemetry.