While Mozilla has not yet enabled Session Restore button on the tab bar in Firefox 56 by default, but you can display the button as the preference now exists in about:config. And in Firefox 57, the Firefox team has moved ‘Restore Previous Session’ item from Library > History to the Hamburger menu.
The Restore Previous Session menu item appears in the menu only when Firefox has automatic Session restore set to off, i.e when Firefox starts with either home or blank page only, but not with a list of tabs and Windows from last time.
Firefox 57 : Mozilla adds ‘Restore Previous Session’ menu item to Hamburger Menu
Once the user uses Restore Previous Session option, it will be grayed out in the menu.
For Firefox users, who have automatic session restore turned on, Restore Previous Session menu item won’t appear in the application menu.
In case you missed this news, recently, Mozilla has landed a change in Firefox 57, to not to display search bar by default for new profiles.