Note that I think this blocking is done not by detecting a site doing it, but instead by using block lists: Firefox has a list of sites where third-party resources, including JS that would do canvas fingerprinting, will be blocked.

As I understand it, this is different from the various "resistFingerprinting" ("RFP") settings in about:config, which will work on every site (and are notorious for breaking things). Ditto CanvasBlocker, which AFAIK runs on every site.

I got this info from one of the links in the informative post you cited: https://blog.mozilla.org/security/2020/01/07/firefox-72-fing...