For example, an Austrian man was arrested in 2011 for running an exit node and charged with being an accomplice to crimes that were carried out over Tor using his exit node. He was ultimately found not guilty, but a law was passed as a result that effectively makes it illegal to run a Tor exit in Austria. [0]
Meanwhile, in the US no one has ever been arrested simply for running a Tor exit node (at least to my knowledge). Anecdotal information suggests that the most difficult thing is finding someone to host the node (many cloud VPS providers, for example, will not) if you don't host it yourself. A Reddit commentator and operator of Tor exits suggests that running Tor exits is protected under U.S. law, although I'm not sure if this has been tested in court [1].
I think Mozilla should take the (relatively small, due to their presence in the U.S.) risk of running Tor exit nodes. They could even turn it into a project of its own, to explore the common problems and develop some best practices for running Tor exits. I could imagine this being a fruitful collaboration with the EFF, for example!
[0] https://www.techdirt.com/articles/20140701/18013327753/tor-n... [1] http://www.reddit.com/r/IAmA/comments/20243q/iaman_operator_...