Firefox browser New Tab Page has been replaced by Activity Stream, which is now landed as system add-on in version 56 and no longer a text pilot experiment. Activity Stream, which Mozilla says helps you ‘rediscover the things you love the most on the web’, displays thumbnails of top sites along with highlights from your bookmarks and browsing history.
For each thumbnail displayed, these options are available in the context menu: pin, open in a New Window, Open in a New Private Window, Dismiss and Delete from History.
The Activity Stream feature schedule to ship with Firefox 56 (update: Mozilla postponed the feature release to Firefox 57), set to display and search bar and top sites, which can be hidden by clicking on the settings shown after clicking on the cog wheel. The hidden preference for AS is ‘browser.newtabpage.activity-stream.enabled’, you can toggle that preference value to attain old New Tab Page, but that pref won’t last for long as we know.
These are the activity stream default sites, which you can find in about: config.
Facebook, YouTube, Amazon, Yahoo, ebay and Twitter. Firefox displays them only when the browser has no history to fetch and display the data, case with new Firefox installations.
FYI, Mozilla is testing an experimental version of Activity Stream with Pocket integration, which you may see in later versions of the browser.