Canonical's new Mir display server, and the discussion/controversy around it, illustrate some of the technical advantages of FirefoxOS.
By supporting nothing but Web applications, we've raised the level of abstraction for our platform APIs so applications are completely oblivious to how cross-process rendering and event delivery are implemented. We can evolve our implementation and our IPC protocols at will without breaking applications. (In fact we're in the middle of doing just that.)
Because Web applications expose a scene graph, we are able to gather scene graphs across processes, and combine and composite them together in the compositor process. Exchanging scene graphs instead of just surfaces lets us do sophisticated effects such as animation and scrolling of application content directly in the compositor. That has some complexity, but it's less complex and more efficient than having apps independently do their own compositing and coordinate with the system compositor.
Then it's interesting to consider what happens when you run Web apps on a non-Web mobile platform. You have to load up a browser stack that's going to look quite similar to FirefoxOS, and run it on top of the native stack and system compositor, much of whose functionality it duplicates. There's significant overhead there compared to FirefoxOS.
Avoiding that duplication also means there's less work to do for us developers. Most of the cross-process compositing work we've done for FirefoxOS is generic work that pays off for Firefox all platforms. In fact, that's true for most of the Gecko work we've done (and will do) for FFOS. For a small organization like Mozilla that needs to punch above its weight, that's critical.
Having said that, in the rush to get FFOS ready for market we've gone out on a limb a bit and created a number of features that only work in FFOS currently. A big theme for our development in 2013 is to get those features working on all the other platforms. This will benefit the Firefox product on those platforms and also make our developers' lives easier.