My battery died & closed all apps including Firefox. Firefox usually restores all tabs but this time it opened to a blank tab. "Recently closed tabs" is grayed. How can I restore tabs from my previous session under this circumstance?

There are no guarantees with this but you may be able to recover them if you have not already closed Firefox again since this happened.

Open the Firefox profile folder, for details of how to find it see the profiles article. If there is a file there called sessionstore.bak, rename it to sessionstore.old

Now close Firefox, delete sessionstore.js in the profile folder and rename sessionstore.old to sessionstore.js

Re-start Firefox to see if your old session is restored.

Unfortunately, I had already closed Firefox again since this happened. I tried anyway but to no avail.

Thanks anyway. I will know this if there is ever a next time.