it's not a browser extension, its a SLAAC thing https://www.internetsociety.org/resources/deploy360/2014/privacy-extensions-for-ipv6-slaac.

TL;DR is that SLAAC used to use part of your device MAC to form it's IP, which would be trackable/fingerprintable. Now devices just pick the last 48-bits at complete random on the assumption that no other device is going to have that specific address out of the 4 quintilion available addresses.

edit the RFC https://datatracker.ietf.org/doc/html/rfc4941