sorry, you are confusing Firefox BROWSER with Firefox OS!
firefox OS is a small linux with geko installed and most of the GUI are webpages. Geko today is light, fast enough and been adding multi-thread.
This is small enough for many application where a easy and fast to develop GUI interface. Being html, you then can point any browser to the fridge and do the same thing. Try that with a android app.
Finally, you will not be opening 30 tabs in firefoxOS, you will probably only see 1 to 3 tabs (if really need) of status/config pages.
You could do this with any distro, but X11/wayland+libs+drivers+sdk is always a problem when compared to a cheap sdk that takes care of that for you.
You could do that with android, but android is harder to change and may be even slower due to all the garbage required for phones, tables.
Mozilla is trying to overshot the mobile, as they learned that is already a market too hard to break in, and go directly to the "internet of things" to increase market share on a future huge market.