What is #Mozilla ads really?

The change is that before Mozilla had a specific list of partners for ads and sponsored bookmarks placed on the "new tab" page.

Now these ad spaces are available on the open ad market to anyone, through their new ad agency and their partners: Equativ and Index.

They claim that no user data is sent to the advertisers, but I don't know if this can be verified: they are acting as a proxy between user and advertiser, and I didn't find the source code of this proxy.