The problem with that theme is not that it's too green. It's that it's too different from the classic Firefox theme, so it might confuse users.
Actually, I find its usability slightly problematic. Some separations between things of the interface just disappear (borders of the drop-down menus, for instance).
So a Mint theme is a good idea, provided it's not too different from the classic Firefox theme.
Anyway, it's still possible to include a greenish theme not as default, but as an option in Firefox.