Mozilla is putting quite a bit of effort into apps right now. "App tabs" in the browser is definitely one small step in getting a nice total ecosystem in place for developers.

FWIW, when I looked at Raven, my initial thought was "it's a browser with app tabs on the side". It's nicely designed, but doesn't seem groundbreaking to me.

I do like the Instapaper integration, though! I also like people exploring new browser ideas.

(obdisclaimer: I work for Mozilla)