Backup json files not restoring.

Hi Mozilla Support, I made a boo boo, was half asleep trying to shut down and wanted to quickly save a set of browser tabs to come back to later, so "attempted" to create a folder to save within a nested structure in my bookmarks library (using Ctrl+Shift+O editor). I right clicked and intended to hit 'f' for new folder in the structure, hit 'd' instead (delete). This was on an upper level folder structure of thousands (10's of thousands....?) of bookmarks. Realised my half asleep mistake and doubled down! Hit Ctrl+Z to undo changes. Well, I guess FF was struggling to process all of the changes, then got confused with the undo, and broke without performing either... F**K. But ok, I will just restore from a backup, I have those setup. So I went to 'Import/Backup -> Restore, and picked the latest date. Okayed the warning and waited. It failed, gracelessly, by deleting all my bookmarks, then errored. "Unable to process the backup file." Now I can't restore any of the restore files in the list. The latest one looks broken with almost no data, but all of the earlier ones are there, they exist in the folder structure under AppData, say they have 30,000+ bookmarks, and are all around the 20MB size. None of these restore. My only hard HTML backup is from early in the year, March, and that is very old in my activity levels. Also, why are all of the json files failing? I've since restarted my PC, slept and come back to the problem awake. Still same problems. I have gone as far as using the 'Refresh' technique in the uninstall options, and despite everything else coming back from scratch, I still cannot restore my json backup files. They all give the same warning, delete any bookmarks I have added and then error with, "Unable to process the backup file." Is this something that has happened before? I get I screwed something up, but what could be preventing this from restoring? Are all of the json files now poisoned somehow? Do I have to do a full uninstall and reinstall and try again? And if that fails, what are my options? I have backups for a reason, this very reason in fact, so that when I error, I don't delete my life... Would love some guidance on this, if there is a way to check .json integrity, or anything else pertinent I might have missed, I would appreciate a poke in the right direction.