This would probably start the trend of the dev features being reduced in regular browsers, I'd guess. But you're right no one is taking it away yet.
Seriously: My guess would be download size, performance, maintenance. Especially with Moz aiming to target super cheap hardware with their OS.
On the other hand, no one would bother with that in a world where non-mobile computers are increasingly becoming developer-only machines, so this is probably just mild paranoia.
With FFOS, Android/Chrome, and MS there's a conscious effort to eliminate the difference in development between desktop, tablet and phone. With more people using computers, not as many people need or want desktop apps and those features on all of their desktop apps. If I suddenly need to edit an image, I don't need PS or even Gimp, I'm often fine with a web app.