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.

Thanks, Tony. I'll give it a shot. I've found the sessionstore.bak file, but also see two files, sessionstore-1.js and sessionstore.js. Just want to confirm that I shouldn't do anything with those other two files - only change sessionstore.bak to sessionstore.old.

Oops...the danger of not re-reading through the full post. Pls ignore...

Nope, it didn't work. What about that sessionstore-1.js file ... anything we could do with that?

You could try renaming that file to sessionstore.js, check the date on it to see if it was created today, it may be an old file.

You're right - sessionstore-1.js is from July. Okay, time to delve into yesterday's history. Many thanks for your help~

The presence of a sessionstore file with a number (sessionstore-1.js) indicates that at one time there has been a problem with accessing sessionstore.js and that Firefox wasn't able to rename sessionstore-1.js to sessionstore.js

You can see by the file date of sessionstore-1.js when that has happened and if it an old file then you can remove it or you can try to copy that file to sessionstore.js to see which session it brings back.

Make a backup copy of all sessionstore.* files elsewhere to be sure that you do not lose a session before doing that.