You can try to restore the upgrade.jsonlz4 file from the last update or use a utility to browse Windows System Restore points to see if you can find a recent sessionstore.jsonlz4 file.

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

You can copy a file with Firefox closed from the sessionstore-backups folder to the main profile folder and rename the file to sessionstore.jsonlz4 to replace the current file.

• make sure to backup the current sessionstore.jsonlz4

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

You can use the button on the "Help -> More Troubleshooting Information" (about:support) to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

It does not work. It only restores those sessions after the problem occurred. I cannot restore an earlier session.

Don't you have an upgrade.jsonlz4 file created when Firefox updates like from updating from 125.0.3 to to 126 or possibly from 125.0.2 to 125.0.3?

I am not sure. How can I check this?