I think this has been this way for a little while, but Breeze offers the ability to take it from the half squared/half rounded window decoration to a full rounded version. You just have to enable the thin border option in the Breeze window decoration settings. (sorry I'm not at a Linux OS right now to say exact name of the setting).

HOWEVER, I discovered that Firefox has included the option to make it's own bottom border rounded on Linux, which up until then was a sore point for KDE themes. To enable it, in Firefox about:config, turn "widget.gtk.rounded-bottom-corners.enabled" to "true" and restart Firefox.

I have no idea how long that FF setting has been there, but I'm really glad it is.

Now we have fully rounded window decorations and a proper looking DE. Thanks!!