Wow, now that’s a very intricate setup. The fact that the windows don’t go to their corresponding activity seems weird, maybe it’s a bug? Seems like it’d be hard to find the cause, though.
Also, when you reboot, do the windows with different firefox instances stay with the same tabs open? Are the profiles kept? Since I prefer to start on a clean slate, I start a new session and simply autostart my usual apps which are bound to their respective desktops. But if even those two are kept, it does seem pretty damn great. Pretty similar to suspending, in a way. Too bad about the activities part though.