The Firefox browser has a secret filter that prevents adult web pages from appearing on the new tabs page, exposing private browsing habits to nearby friends, strangers and family.
The filter is not new, it is dated by 2014, but was recently detected by some Redditors.
In its current form, the filter prevents the thumbnails / tiles that lead to adult sites from appearing in the Top Sites and Highlights section of each new Firefox tab.
This means that if you are accustomed to visiting such websites (intentionally or completely randomly), these pages will be hidden and will not appear when you open a new tab in Firefox.
The reasons behind this filter are not the privacy of users but purely commercial.
The filter was first developed and applied to 2014. At that time, Firefox's New Tab contained a section called Sponsored Tiles, which was used as a way of displaying ads as well as for viewing some of the pages frequently visited by the user.
Mozilla's developers added the filter because their content buyers do not want the tabs next to contain content from adult pages.
Sponsored Tiles has finally been removed, but the adult filter has remained and is now part of the new Firefox New Tab page.
The feature is turned on by default, but you can disable it from the Firefox settings page:
Open the about: config internal address and look for the "browser.newtabpage.activity-stream.filterAdult" setting. If you set the value to false, you will see the adult tiles again.
The blocked filter list contains 2.919 URLs encoded in Base64 MD5 hashes.