Good question, never gave this any thought. see screenshots You will have to look in your profile for a folder called sessionstore-backups. That's the folder in my screenshot and you see the file in the red box. I could make the changes mentioned in the pic and remove the current sessionstore.jsonlz4 in my profile folder and go back to my session from the date in the pic.

So go to your profile folder from the about:support page and then the sessionstore-backups and see what the oldest date is. Post screenshots if you have questions.

You may have a upgrade.jsonlz4 in the sessionstore-backups folder from around that time and that is as far as you can get.

You can look at this tool to inspect a compressed jsonlz4 sessionstore file. This tool works locally, no uploading done.

• https://www.jeffersonscher.com/res/scrounger.html