I was trying to disable telemetry on firefox after having refreshed it (it wiped out everything and reset everything)
And I found this github page to be very concise and to the point. If you are planning on disabling telemetry, this might help https://github.com/K3V1991/Disable-Firefox-Telemetry-and-Data-Collection
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Arkenfox is the defacto gold standard in hardening firefox
https://github.com/arkenfox/user.js/
But if you just want it all set correctly out of the box Mullvad Browser