Starting with v95 Firefox supports GCP:
https://globalprivacycontrol.org/faq
https://blog.mozilla.org/netpolicy/2021/10/28/implementing-global-privacy-control/
You still need to set privacy.globalprivacycontrol.enabled and privacy.globalprivacycontrol.functionality.enabled to true yourself.
It looks like this is just yet another header, but with different bureaucracy behind it. (Sec-GPC)
NB: The current ESR is v91.
