They could charge for services such as sync but again that is going to be difficult Chrome does it for free.
I think Mozilla need to seriously look at how they are spending their money as it seems to be pretty insane some of their outgoings.
I know they are trying to diverge from just being a browser with things like Firefox OS but we all know that isn't going to ever be a real competitor to iOS and Android. Even in developing nations I think people are more likely to go with a dirt cheap Windows Phone when Windows 10 is out over a Firefox phone. One thing Windows Phone does really well is run great on very low powered devices. I can't ever see Firefox OS getting above 0.x market share.
Perhaps Mozilla is just too big now? Do they need to scale back to save money? Can they survive on a donations alone?