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.
Thank you for providing your idea and feel free to ask any questions meanwhile.
I should mention that there is a "standard" scaling for circular macOS icons within Apple's design guidelines, which both Safari and Chrome already follow. I hope Firefox's macOS icon will be scaled the same — as it will fit in with other icons on the platform. Here is an image demonstrating the alignment:
Standard sizes for circular icons in macOS 11+ are as follows:
actual resolution
@1x
@2x
circle width / height
1024x1024 px
N/A
512x512@2x
710x710 px
512x512 px
512x512@1x
256x256@2x
356x356 px
256x256 px
256x256@1x
128x128@2x
178x178 px
128x128 px
128x128@1x
N/A
88x88 px
64x64 px
N/A
32x32@2x
44x44 px
32x32 px
32x32@1x
16x16@2x
20x20 px
16x16 px
16x16@1x
N/A
10x10 px
Also, after testing various background colors for the icon on different wallpapers and within the context of other macOS icons, this gradient (composed of three colors from Firefox's brand palette — #9059FF, #7543E3, & #582ACB) feels by far the most natural imo:
I’m on Firefox Developer Edition and already have the new icon. However, it’s smaller than the other icons. It’s kinda in an uncanny valley territory because it’s close to the right size, but not quite. Really visually annoying and I’d strongly prefer the old one back or the size to be fixed. It’s smaller both in the dock and in the Command-Tab screen.