That's what https://support.mozilla.org/en-US/kb/how-do-tiles-work-firefox#w_what-data-is-being-collected-and-why says but https://bugzilla.mozilla.org/show_bug.cgi?id=1120311#c0 says "take the top 100 links data already available from NewTabUtils and remember which sites to match against" and later describes some measures to make it hard for Mozilla to guess what sites a user has actually visited based on what suggested tile is shown. I suspect the support page is wrong and should be corrected as it gives the impression of vast data collection by Mozilla, and then unwanted to dangerous leaks of browsing history to others who happen to be using the same IP. I could be wildly misunderstanding things! Thanks in any case... (I tried to use the discussion forum for the support page in question but when starting a new thread or posting a reply I was redirected to the support home page. Edit: probably because I hadn't confirmed my email address yet. Text above now at https://support.mozilla.org/en-US/kb/how-do-tiles-work-firefox/discuss/5250#post-11810 also.)