Firefox browser New Tab Page has been replaced by Activity Stream, which is now landed as system add-on in version 57 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 under Top Sites, these options are available in the context menu: pin, edit, 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 57, set to display search bar, top sites, highlights and snippets.
Highlights shows the pages you’ve visited or bookmarked recently ( limiting one to per domain).
Snippets: After completing the onboarding tour, Firefox shows updates from Mozilla.
These can be removed by unchecking the sections under New Tab Prefrerences, click on cog wheel and choose what you want to see on the new tab page.
The hidden preference for AS is ‘browser.newtabpage.activity-stream.enabled’, you can toggle that preference value to false 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, Reddit, Wikipedia 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 has already tested experimental version of Activity Stream with Pocket integration, which you may see in later versions of the activity stream.