I recently upgraded my browser to FireFox Quantum and found most of my addons no longer worked as the devs haven't updated them yet. One thing I forgot I missed until I needed it was the Restore Closed Tab toolbar button I had with the Session Manager addon. The way this worked was that when I accidentally closed a tab I could press the toolbar button and the tab would re-open along with it's back/forwards history. Turns out there is a built in FireFox way to do this. Just press CTRL-Shift-T and the tab will open again.