Here is more information (linked from the article) about how the sponsored stories are personalized while respecting user privacy. The client will download an unpersonalized list of sponsored stories and then filter the list on the client-side by comparing it to the user's browsing history. So the user sees relevant sponsored stories without Mozilla or publisher servers seeing personally identifiable information.

https://help.getpocket.com/article/1142-firefox-new-tab-reco...