Things have changed slightly. Here is what I have in the sessionstore-backups folder.

The previous.js file is rather large, so make a backup of that file just to be sure.

Try to copy that previous.js file to sessionstore.js in the main profile folder.

You can check History > Recently Closed Tabs/Windows

How do I get to the main profile folder? I haven't seen sessionstore.js anywhere.

Edit: Nevermind, I'm in the folder, but still no sessionrestore when FF is closed

So I copied the previous.js from the sessionstore-backups folder and pasted it in the main profile folder and renamed it to sessionstore.js, but restore previous session is still grayed out.

Firefox would open the session stored in that sessionstore.js (previous.js) file automatically without the need to restore the previous session if there isn't a problem with this file.

Did you check History > Recently Closed Tabs/Windows to see if there are closed tabs or windows?

You can open the file in the Scratchpad ("3-bar" menu button or Tools > Web Developer) to see if it contains the lost tabs. Click the "Pretty Print" button to format the file for readability.

Recently Closed Tabs/Windows didn't show up when I clicked history. But when I open the most recent update.js and the previous.js I see the URLs. I opened scratchpad, but I don't think it is working for me. Nothing I click does anything.

EDIT: I figured out the problem. My Firefox reverted back to the previous version. As soon as I updated it again update.js worked and my tabs are back. Thank you for your help.