A brilliant concept, with brilliant results. Only problem is, sometimes, the logos in the websites change (which is obviously the intention), but the thumbnails on the tabs and bookmarks bar do not change (or even vice-versa). I hope that these issues are quickly fixed, since I find this addon to be truly underrated, as someone who has got used to, but still doesn't really like the look of the September 2020 icons.

An excellent extension/add-on, works well in Chromium Edge and Firefox.
One (very) minor issue I've just noticed is, in Gmail Basic HTML, the new colourful Gmail icon is still shown (though the favicon is corrected to the old one); please fix this.
Overall, quite good!

Extension is painfully simple and backdoor free. thanks for the effort.

edit: here is his github: https://github.com/dangeredwolf/classic-google-icons

Since today missing the classic icon for calendar.

Thank you!