Update: I've noticed this is getting many page views lately. Unfortunately, it no longer works. Google learned about leaving their old icon in the servers and changed it too. The only solution I can think of is hosting the old icon somewhere else and changing the url in the following script to use that hosted icon.
Yesterday gmail changed the favicon.
I don't like the new one. I wonder if the designer has ever witnessed the effects of lightning in a real life envelope. Shadows don't do that :).
This is serious business for me, because I am in front of a computer most of my life. And most of the time I am in front of a computer, it is looking at my browser window. And most of the time I am looking at my browser window, the gmail icon is the one in the first pinned app.
So here is the link to the userscripts.org entry: http://userscripts.org/scripts/show/169207. And here is the script itself:
// @name old favicon
// @namespace nouglyenvelopeshadow
// @description Changes gmail's favicon back to the old one
// @include http*://mail.google.com*
// @version 1
// @grant none
var link = document.createElement('link');
link.type = 'image/x-icon';
link.rel = 'shortcut icon';
link.href = '/mail/u/0/images/favicon.ico';