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.
Jon, the Community Manager, shared exciting news that the project is now "In development" for Monkey Mart. He encourages everyone to ask questions and stay engaged for further updates.