Just fits in better.
It's not a big problem but it doesn't really fit in.
Users on Android 7 and below, and those on Android 8 and above with launchers that use legacy icons.
What do you mean by "legacy icon"? A screenshot would help!
Android 8 introduced adaptive icons. Google refers to shapeless icons as legacy icons. Here's a comparison of Firefox (Fennec) vs Firefox Beta (Fenix)

Thanks!
The now discontinued org.mozilla.fennec_aurora package had the proper non-adaptive Nightly icon for example, whereas org.mozilla.fenix suffers from the described issue.
@NotWoods do you know anything about these legacy icons and if we have the right assets?
We can probably just resize the assets from mozilla.design, but maybe @shorlander has appropriately sized and padded transparent icons for Android already?
Adding screenshots.
Proper older icon (Firefox 68.11.0): https://i.imgur.com/GyrXUCg.jpg
Current "Fenix" messed-up icon:
https://i.imgur.com/ZpWIP2S.jpg
Regards.
Looks like this also affects the release channel. I've kept my release install on fennec so I didn't know.
No one?
Please try to use Firefox on Android 5.1.1, so you can see what's we're talking about.
The icon size is currently ugly, very small and no transparency.
Icon is part of the branding, so this is not trivial matter.
Most helpful comment
The now discontinued
org.mozilla.fennec_aurorapackage had the proper non-adaptive Nightly icon for example, whereasorg.mozilla.fenixsuffers from the described issue.