Imagine if your browser history was local but every time you use the history feature to revisit a page that URL was sent to Mozilla. How private is that really?

The Pocket code tells Mozilla what ads are being shown and which are clicked on. Since they know that the criteria for ad X is that phrase Y or Z is in the history, then they are gathering data on the type of content you are looking at on the web. This is only one step removed from the actual URLs.