I understand that Nvu comes from the Mozilla Composer code base.
First impressions. That Nvu is derived from Composer is very obvious. Composer's layout is copied almost completely including the tools and the ordering of the tools on the toolbar, the tabs at the bottom for switching between editor modes, and the status bar at the very bottom.
To that, Nvu has added some nice features, including the "rulers" that lets you know the dimensions of the editor window and, especially, the site manager. There are also some new tools added to the toolbar which are added at the end of those copied from Composer along with the addition of a third toolbar. There are also some additional options in some of the menus.
And, it doesn't have the baggage of the other Mozilla modules (hence, the lack of icon links to those modules in the status bar).
I have not yet had time to actually use Nvu - maybe this weekend - but it does look like a strong evolutionary and improved form of Composer.