Back then, the Firefox New Tab page contained a section called Suggested Tiles, which was used as a way to show sponsored content and to surface some of users' most visited sites. The filter was developed and implemented back in 2014. The reasons behind this filter don't reside in the engineers' attention to protecting users' browsing habits and privacy, but in sheer commercialism.
This means that any time a user makes a habit of visiting a particular adult site, or the user intentionally or accidentally bookmarks an adult page, that page won't show up on the New Tab page at the most imappropriate times. In its current form, the filter prevents thumbnails/tiles for adult sites from appearing in the Top Sites and Highlights section of the Firefox New Tab page. The filter isn't new, dating back to 2014, but has only recently been spotted by some keen-eye Redditors, in November 2017 and today. The Firefox browser includes a secret filter that prevents adult sites from showing up on the new tab page, and by doing so, exposing some users' unsavory web browsing habits to nearby friends, to shoulder-surfing strangers, or in the worst case scenario - to the media. Google Drive alternative: Decentralized and encrypted
