As we reported some time ago Mozilla has added session restore button to the tab bar in version 56 of Firefox browser, but disabled by default. If you’ve not changed Firefox startup behavior to ‘show tabs and windows from last time’ and using the default ‘show your home page’ setting Mozilla browser will display ‘Restore Tabs from Last Time‘ button in the tab bar. Considering the usage, the Firefox team may make automatic session restore as default startup setting, which may not happen right now.
Firefox 56: Restore Previous Session from tab bar
Here is how to enable session restore button in Firefox 56, you should be using latest Nightly version for this.
1. Visit about:config
2. Find and change browser.tabs.restorebutton preference value to true. And you’re good to go.
When you open a new tab the session restore button will disappear, and it will only appear when one tab is open (that is Firefox home page).