Note: Some versions of Quantum change how the data files are set up, so they are not backward compatible.

See if this can help;

http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

jscher2000 has a tool on his website to extract out your bookmarks and save them to an HTML-format bookmark export file. If you want to try it, here is the page:

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

To open the bookmarkbackups folder in Finder, use the "Open in Finder" button in the first table on the Troubleshooting Information page, and double-click into bookmarkbackups where you should see several dated files. From there you can drag and drop onto the conversion tool.

To import the HTML file, see the steps in this article: Import Bookmarks from an HTML file.

Back up and restore information in Firefox profiles.{web link} I use an add-on that makes backups of my user profile, Just In Case. Unfortunately, it is not compatible with the current Firefox.

Every now and then, open the profile folder and create a copy of the current profile as is to another location on your hard drive. This way you will have a backup.

If you do this each day, you will not lose much data. This should be done with Firefox Closed.

Note: In case you need to restore from these backups, you may need to overwrite old or corrupted files.

Thank you Fred. I really appreciate it.

Unfortunately, I had immoveable deadlines to attend to, and it took a week before I could get to this. In the mean time, all but one of the backups rotated out of existence.

The JeffersonScher tool succeeded in getting an html file out of the .jsonz4== file but so far I haven't succeeded in getting Firefox 52.1.2 to inhale the .html file. The "Show all bookmarks" page is still blank. So I am going to uninstall Firefox as completely as I can, and see what happens next.

I hate software updates. Hate 'em hate 'em hate 'em. The "weekly update" culture of Firefox is by far its worst feature.

Note the 52.1.2 ESR was a old version from May 19, 2017 when the last security/stability update for the old EOL Firefox 52 ESR was 52.9.0 ESR released on June 26, 2018.

http://archive.mozilla.org/pub/firefox/releases/52.9.0esr/win32/

https://www.mozilla.org/firefox/releases/

If you must use a EOL Version at least use the last update that supported the old EOL Windows XP (and Vista) which was 52.9.0esr.

Thank you. That is important to know.

Note that you can't downgrade a profile used by Firefox 55+ to an older Firefox version. Some file were upgraded to newer versions and are no longer compatible and other files were added. The key4.db file has replaced key3.db as the key file for the Password Manager (passwords are still stored in logins.json). The cert9.db file has replaced cert8.db for intermediate certificates. The favicons are stored in a separate favicons.sqlite database and no longer in places.sqlite.

You should at least clear/remove some files and folders (keep a backup) to undo changes made to some files.

• places.sqlite and favicons.sqlite to undo changes made to the bookmarks and history storage.
  Firefox should rebuild the bookmarks from a backup in the bookmarkbackups folder
• storage.sqlite and serviceworker.txt and the storage folder
• key4.db and cert9.db (SQLite versions of key3.db and cert8.db)
• pkcs11.txt that replaced secmode.db
• handlers.json that replaced mimeTypes.rdf
• addonStartup.json.lz4

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

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox