I also agree that the new macOS dock icon for v106 looks very bad. I also joined just to provide my feedback because of how bad it looks. I think it needs a white/lighter background and agree that as it is now there's an illusion that it's smaller than the other icons in my dock.
I ended up retrieving firefox.icns from a backup, and dropped it onto the up left corner of the dialogue resulting from Get Info on the Firefox app. Problem temporarily solved.
Checking from a Monterey Mac, I must say the new icon looks better than on Catalina. Still, I would say a light-background icon like the one in the original suggestion would look better than the current one.