I use Firefox, I have GTP in the favorites bar, and lots of times the favicon changes. Its always the same logo but a bit different. For xample sometimes it has a black square around the circule and other times its transparent. Also sometimes the circle is less black and more grey. Onc ein a while the logo shows a bit pixelated aswell. Minor stuff like that.
Well, the site provides favicons in all standard formats, so web browsers can use whichever format they need. Why Firefox would switch between those different icons is a mystery to me.