When I tried to save 1'991 open tabs, this damned extension literally closed all my tabs when I clicked "Shopping" to see how it worked, and it didn't save them. Firefox's history didn't help, and now I have to search for 1'991 tabs again.
I don't recommend installing it!

this works, but seems to create an issue with a lot of popup windows, including a lot of google login windows and corporate login popups.