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.

A few months ago, the favicon for Walmart changed from the yellow Walmart asterisk to the generic black and white globe. I was hoping that this app would fix that but it didn't.

Edit: I saw that I needed to click on the app's icon and select "Reload Page to Retrieve Icons" button and it adds the site's icon for all the bookmarks that had the generic black and white globe.

This add-on addresses an issue I've had for ages with Firefox. It works great. I've run into a few sites that don't seem to respond, but the vast majority now save the favicon like they should. Thanks!