It's great to hear your thought on updating the Firefox icon(s).
Our team is currently working on it, and all the Firefox browsers will soon have a square-round icon for macOS. This will meet the Apple icon guideline.
Thanks @Jon for the quick work. PiP team has been working on new design and like Ania mentioned it will be great to have some volunteers for user testing in a few months. Stay tuned ✌- Firefox UX Team