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 implemented in 2014. At that time, the New tab του Firefox περιείχε ένα τμήμα που ονομαζόταν Sponsored Tiles (σε ελεύθερη translation Προτεινόμενα πλακίδια), τα οποία χρησιμοποιούνταν σαν ένας τρόπος projectionof advertisements but also for the display of certain pages that were frequently visited by the user.
The developers of the foundation Mozilla they added the filter because their commercial partners who bought content don't want side tabs 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 function is enabled by defaultchoice, 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.