Firefox has a secret porn filter that prevents adult sites from showing up on your new tab page

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.

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.

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.

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.

The reasons behind this filter don't reside in the engineers' attention to protecting users' browsing habits and privacy, but in sheer commercialism.

The filter was developed and implemented back in 2014. 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 original adult filter list wasn't the only blacklist that Firefox implemented for the New Tab page back in 2014. Devs also included other blacklists, like the one for preventing thumbnails of piracy and torrent sites from being shown next to tiles promoting official DVDs.

"MGM would not want a 007 DVD release to be appearing within the same page (in particular, directly next to) where PirateBay tile appears," said a Firefox engineer back in 2014.

But while the Sponsored Tiles feature was eventually removed, the adult blacklist remained, and is now a part of Firefox's new New Tab page [1, 2, 3] --revamped in November 2017 with the release of the new Firefox Quantum engine.

The blacklist is enabled by default, but users can turn it off by accessing the Firefox about:config settings page, searching for the "browser.newtabpage.activity-stream.filterAdult" option, and setting the value to false.

The actual list can be consulted online here, or inside a Firefox browser by accessing resource://activity-stream/lib/FilterAdult.jsm. However, users won't understand much of it, as the list isn't human readable, containing 2,919 URLs in Base64-encoded MD5 hashes. [Update: Some of our readers decoded the list. Besides adult sites, the list also includes drugs and gambling-related domains. The decoded list is available here and here.]

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