All visited URLs should be in your History (Ctrl+H) unless you disabled it or used a private window.

If you use a lot of windows, you may want to increase the undo limit (3) by going to about:config and changing browser.sessionstore.max_windows_undo.

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.

• https://www.shadowexplorer.com/
• https://www.nirsoft.net/utils/shadow_copy_view.html

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page 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

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

• 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

There was no way to get the data back. The history was cleared externally by Spybot. It was not expected that this would apply to the active windows of my Firefox session. I was attempting to debug another problem at the time...

Increasing my window undo limit from 3 to 50 has been useful since then, so while there is no real solution to my problem, I'm going to mark zeroknight's response as the solution to close it out.