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.
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.
The new icon looks EXTREMELY UGLY on my macOS Catalina.It is the only icon with non-transparent black background. I have other apps that have rounded square icons, but none of them looks so ugly.
