I've just checked, and 22 out of 29 Dock icons on my Mojave system are not the standard rounded-square. My opinion is that different shapes, like differing colours, can be an aid to quickly identifying the icons.
This has already been posted to the Bugzilla group, Square icon for macOS Big Sur but the thread has since been closed as there was no value added and it seems it's not a huge priority for them based on my contribution and following of the thread.
That's odd. I just looked and had three custom icons. I removed two (old) ones and left Firefox. I never notice these icons as different when looking for something and wouldn't be able to have told you who or how many I had, other than obviously one was Firefox. Put me down as the silent majority(?) that don't care.
@edumorlom, It's just an icon and I'm sure they have other priorities.
Check out MacOSicons.com and search for Firefox, click the icon you want to download and then go to Firefox in your Applications folder, right click on it and select Get Info. Then drag the icns file you just downloaded to the current icon in the get info window. Problem solved.