At the moment, Firefox already blocks some cryptojacking scripts via the Firefox Tracking Protection, a feature that blocks abuse ad trackers. Two domains are blocked, those of Coinhive and Crypto-Loot, these account for well over 80 percent of the cryptojacking market. However, this feature is only activate by default when you open a private browsing window.
Here's a look at what the more extensive cryptojacking protection may look like:
Discussions on this possible feature is already taking place on the Mozilla bug tracker, here.
The other alternative would be if this anti-cryptojacking protection would be part of Firefox's upcoming "ad filter system" that Mozilla plans to add to Firefox this year, to protect users from today's toxic advertising landscape. More details on that, here.