I've also just realized that I have several extra copies of folders, some with many of my other bookmarks (also mixed up) inside them.

Are you using any extensions that affect bookmarks?

This user had a very similar issue:

https://support.mozilla.org/en-US/questions/1107586

And this solution seemed to work

Do you use Sync in Firefox or an add-on that would affect bookmarks? I can only see your Plugins in the details, so Extensions are missing.

If Sync gets confused (bad timestamps, for example) you may get a little bit of mayhem in bookmarks. The solution here is to back up bookmarks manually, turn Sync off, reset/change Sync password and re-enable Sync. This will scramble the encryption key and delete all previous Sync data.

Additionally, turning off bookmark sync seemed to help/work as well

If you use Sync then such a corruption is in most cases always caused by Sync messing with the bookmarks or getting confused about the location. This can especially happen if you have mobile devices connected. Sync seems to store the bookmarks as one long list and can then re-add bookmarks from Sync storage if it thinks that this bookmark isn't present. Having multiple copies already can cause additional problems.

cor-el, that definitely makes sense. I have been using sync for almost a year, though, and it's never done this before. Could it still be a sync problem?

evhenry, it's a good idea. Unfortunately, my experience is dissimilar to the person you referenced. The folders let me open them, bookmarks remain consistent within folders, and no folders have disappeared - but it looks like a folder and its attendant bookmarks have duplicated themselves several times.

I got a notice from Norton a while ago about something like a ttyfn or a tytrn or something like that (sorry, I'm not sure how to find it again). It seems like one of the filenames it referenced was Firefox, or maybe Mozilla. Could these phenomena be related?

My issue is similar but appears to be the Bookmarks Toolbar that gets scrambled, easy to restore from a backup. but it happens everytime I shut down and restart my PC, ie. daily. I have a new mobile which uses Firefox sync, it was all setup from an online backup, never was a problem on my previous Motorola G6 Plus, the new one is the G8 Plus so I would have expected no change in how Firefox behaves. I'm going to try disabling sync on all devices, then restore my toolbar and see how it behaves over a few days.

Gordon,

On laptop / desktop Firefox open Help > Troubleshooting Information and scroll down to Places Database then use the Verify Integrity button. Read the Integrity report when it appears to see what Tasks were performed.

Then restart Firefox.

That should fix any corruption of the places.sqlite file, but it won't fix any corruption that made it into the Sync server or onto "Mobile" Firefox versions.

Update:

I chatted with someone from Norton. He did a remote session and cleared some info. He seemed to think the bookmarks thing happening just before the Norton warning was just a coincidence. Firefox has been working for the last couple of days, so I'm hoping the issue is resolved.

Thanks to everyone for your help!