As pointed out by our friends at BleepingComputer, the Firefox roadmap on Mozilla's official wiki lists the development team's plan to implement what it refers to as "a simple ad filtering option." Though the roadmap states that this option is still in the early stages of its development, the plan is for it to release in the third quarter of this year.
Alongside this ad blocking functionality, Mozilla also plans to add more protection against cross-domain tracking, more refined general tracking protection that allows users to disable specific types of trackers, and consolidating key privacy controls into one location for users to navigate to.
All of these features for Mozilla Firefox are slated for Q3 2018 as of the date of this article's publication. As BleepingComputer notes, the wording on Mozilla's roadmap for Firefox shys away from using the words "ad blocking" in lieu of "ad filtering" in order to emphasize that it will attempt to only disable malicious and misbehaving advertisements.