Ubuntu MATE 16.10 is, more or less, a re-working of Ubuntu MATE from scratch, not just to accomodate GTK3+ but to also make most of the packages shipped by default with Ubuntu MATE “Recommended”. This means most default applications can now be uninstalled without issue. The work to port MATE Desktop to GTK3+ has been ongoing for a couple of years and Ubuntu MATE is the first major distribution to ship a full GTK3+ implementation of the MATE Desktop. And the absolute latest release too, MATE Desktop 1.16! Firefox and LibreOffice are also GTK3+ only in Yakkety.