Hello click on the 3 line menu (also known as the Hamburger Menu) and then click on Restore Previous Session -- it is the 4th menu item on the list.

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file (make sure to backup the current sessionstore.jsonlz4).

Firefox 56+ releases compress the files in the sessionstore-backups folder and sessionstore.jsonlz4 with LZ4 (.jsonlz4 instead of .js), so it is no longer easy to inspect them.

Firefox can still restore uncompressed sessionstore files, but to make this work you need to remove (rename) compressed files and rename the sessionstore-backups folder to ensure that Firefox can't fallback to compressed .jsonlz4 files.

To keep your options open, please back up everything in the sessionstore-backups folder to a neutral location Firefox won't overwrite. To preview the contents of a file, you can drag and drop it onto this tool:

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

Thanks for the offers of help. I managed to restore my tabs using the upgrade.js file from the sessionstore folder. If I'd used the recovery file from the main profile folder, I would've got a more recent set of tabs, but this option disappeared after I restored with the upgrade.js file.