The hard stuff isn't the window system/widget integration, it's having multiple processes open for the same domain (or iframes in different processes) and mediating access to local storage/IDB/cookie jar/network cache/etc.

It was only very recently that WebKit2 handled most of this stuff -- up until then they just had a single WebProcess (I think Safari on Mavericks is the first multi-WebProcess browser Apple have shipped). So it's a hard problem and isn't due to some lack of competence that Mozilla have been slow to adapt. FxOS is fully multiprocess afaik.

Actually getting the rendered page image into a window owned by another process is easy: windows and X let you host HWNDs and Windows from other processes (if you choose to allow your WebProcesses access to the window server) or you could draw into a shared memory segment. The hard stuff is all of the regular browser hard stuff.