Don't panic!
I had something very similar happen to me a couple of days ago. First noticed it when I went to a site I use regularly and Firefox stores the password for that site (nothing radically important!) but this time Firefox didn't deal with it. I then found I had no remembered sites, log ins, passwords etc.
Looking in the profiles directory, the file "logins.json" had been renamed to "logins.json.corrupted" and was useless. Fortunately, I had copied the original file elsewhere and was able to just paste it back to replace the corrupted version and all was well. I remember this having happened some time ago and the answer then was to do a Windows restore back to date prior to the latest Firefox update. Then I copied the .json file.