As we reported some time ago, Mozilla added the restore session button to the tab bar in version 56 of the Firefox browser, but disabled by default. If you haven’t changed Firefox’s startup behavior to ‘show tabs and windows from last time’ and using the default setting ‘show your homepage’ the Mozilla browser will display ‘Restore guides from last time‘ button on the tab bar. Considering usage, Firefox team can do automatic session restore as default startup setting, which may not happen now.
Firefox 56: Restore previous tabbar session
Here is how to enable session restore button in Firefox 56, you must use the latest version of Nightly for this.
1. Visit about:config
2. Find and change browser.tabs.restore button preference value for real. And you’re good to go.
When you open a new tab, the session restore button disappears and will only appear when a tab is open (which is the Firefox homepage).
Firefox Nightly gets lazy session restore feature