Thankfully Mozilla works with the Tor Project so that Tor Browser patches to Firefox get uplifted to mainline Firefox, you can read more about those efforts her

Thankfully Mozilla works with the Tor Project so that Tor Browser patches to Firefox get uplifted to mainline Firefox, you can read more about those efforts here: https://wiki.mozilla.org/Security/Tor_Uplift https://wiki.mozilla.org/Security/Tor_Uplift And the Tor Uplift tracker: https://torpat.ch/uplift https://torpat.ch/uplift In this case the relevant preference is privacy.firstparty.isolate = true. Another worth pointing out pref is privacy.resistFingerprinting = true.