Tested and works good on https://mbasic.facebook.com/ but not for https://mail.google.com/mail/u/0/h/18clw8g8rdnwt/?& aka gmail basic html or maybe any gmail. But that is probably different issue with gmail, because I noticed firefox blocks first favicon in network requests, but not 2nd, and your extension's button is grayed out for this. But probably bug with accept header, when I change it from accept webp to png it works I think. Also I think Firefox a week ago fixed this issue so this extension is no longer needed. The bug I reported in bugzilla is fixed.