For many of us, our browsers' new-tab pages are something of a liability. Whichever browser you use, they all follow a fairly similar style: a bunch of boxes linking to the sites that we use and visit regularly. This is great when your regular sites are Ars, Gmail, and Twitter. But all too often, sites of a less salubrious nature find their way onto our new-tab pages, disclosing to the world our dirty habits when nobody's watching. While we can, of course, clean up our new-tab pages by Xing out the buttons for the offending sites, a moment of inattention can all too easily expose our pornographic predilections to the world.
But one browser is working to protect our secrets: Firefox. A redditor spotted (via Techdows) that Firefox contains code to spare your blushes. The browser contains a hard-coded list of adult site domains, and if one of your most-visited sites is one of those domains, it will automatically be hidden from the new-tab page. As long as your porn viewing is reasonably mainstream, you never need to worry about Firefox spilling the beans.
It turns out that this isn't actually a new feature. Much like Chrome's advanced tab management capabilities, it's an old feature that's been newly spotted. Mozilla added the code to the browser about four years ago. It wasn't actually created to prevent potential new-tab page embarrassment; rather, it was to aid Firefox's commercialization efforts. Mozilla experimented with having sponsored content on the new-tab page, allowing companies to pay to have their sites promoted in those buttons. Many advertisers don't relish the thought of having their precious brands juxtaposed with Internet filth, so the Firefox developers added the blacklisting capability to try to prevent porn from appearing alongside sponsored content.
Porn isn't the only thing that's blocked; high-profile piracy and torrent sites are similarly blocked, lest a new movie end up being promoted alongside a site offering a bootlegged version of the movie. Overall, just shy of 3,000 domains are on the blacklist, though exactly what they all are isn't immediately clear: the domains are stored in a hashed form, so there's no straightforward way of identifying all of them.
Even though Mozilla's monetization plans have changed over the years and suggested tiles are no longer part of the browser, the blacklisting is still there. It now prevents the forbidden sites from being included in the "top sites" and "highlights" buttons on the new-tab page.
If you're unapologetic about your browsing habits, you can disable the blacklisting through Firefox's about:config settings page. The setting browser.newtabpage.activity-stream.filterAdult can be used to disable the filter.