The icon for "default" isn't a box that looks unfinished. Reference browser and Fennec uses a better icon here.
The icon is a box that looks like a placeholder.

@topotropic is this something that we want to do before release? if so please move to the bugs for release column, otherwise can leave here.
I want to add that since this issue was filed the label ("Default", "Private") was removed so it's even more an issue now. Now it looks like a (non-working) checkbox.
I hope we can fix this issue through https://github.com/mozilla-mobile/fenix/issues/10849
I consider this fixed with https://github.com/mozilla-mobile/fenix/issues/10849
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I want to add that since this issue was filed the label ("Default", "Private") was removed so it's even more an issue now. Now it looks like a (non-working) checkbox.