Firefox makes it easy for you to access great content the moment you open a new tab by displaying your most frequently and recently visited sites along with popular web content. The New Tab page is the default Firefox start page unless you have set a different home page.
When you first use Firefox you'll see links to top-ranking websites and popular search engines. These are eventually replaced with Top SitesShortcuts, which are links to frequently and recently visited sites from your browsing history.
Learn more on how the New Tab page can be customized: