TL;DR: With Firefox 56, Mozilla combined Firefox Health Report and Telemetry data into a single setting called “technical and interaction data”, which was then enabled by default. This included data about advertising within Firefox’s New Tab page, along with a lot of other technical information about the installation of Firefox. The Firefox preferences UI makes no mention of usage of this technical data for advertising purposes. Mozilla has also recently shared that they share data with their advertising partners “on a de-identified or aggregated basis”. Mozilla has given no indication that this is a new occurrence.