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

Works like a charm!
Would you consider extending this to the icons under Google Apps (the icon next to user icon)? The new icons make it disorienting to switch apps there ;_;

Thank you!