Did you rename a file named places.sqlite? If you still have the old one, you could try to resurrect it, since it stores not just history but all your cookies bookmarks. If you find the file, we can exchange ideas on what to do with it. For example, you could rename your current places.sqlite file and then copy the old one back, create an extra backup of your old bookmarks, then close Firefox and switch the places files back, then restore that backup.

That's exactly what I did! I had renamed the old one "places.sqlite.old" as instructed. I now changed the old one to be the active file (regular name) and temporarily saved the newer "places.sqlite" as a different name for the moment. This puts me back in the same situation I had before, not able to view bookmarks or history. When I start Firefox, a red banner notification appears below the tab bar saying "The bookmarks and history system will not be functional because one of Firefox's files is in use by another application. Some security software can cause this problem."

I tried to run Firefox in safe mode, but I still get this error message. I'm a little lost from there on what to try.

What are the file sizes of those JSON backups in the bookmarkbackups folder?

Do you see any URLs in them if you open such a file in a text editor or in Firefox (File > Open File) after adding a .txt file extension to a copy of them?

If you use the Time Machine then you can try to restore older copies of places.sqlite or older JSON backups.

Largest is 4kb, smallest ones are zero. When I open them up as text there is only one bookmark, one that I added right before I noticed the rest were gone.

Think there's a way to solve the original problem (with the original places.sqlite?) without discarding all of my bookmarks?