The way I understand it, the tiles are based on your browsing history but this data is not send to Mozilla. With Suggested Tiles activated, the browser will fetch all available suggested tiles based on country and language, and will then decide which tiles to show.
Herman added, "With Suggested Tiles, we want to show the world that it is possible to do relevant advertising and content recommendations while still respecting users' privacy and giving them control over their data."
That said, Firefox does send your browser Mozilla interaction history with the Tiles feature. Once there, your raw data is stored in the system's storage and analysis engine, Disco. The aggregated data is then saved to a data warehouse, Redshift. This data is then used to create high-level aggregate reports for advertisers.
Specifically, a Mozilla representative said Mozilla collects the following data:
Language preference Tile ID How many times the Tile was displayed Where in the grid of tiles a Tile was displayed What interaction the user has with a Tile: "Rolled over" "Hovered over" Pinned Blocked Clicked Moved