If this is not the Firefox sync program that is included with the Firefox program, then this is possible.

Please try disabling this sync program.

No, its the one that came with Firefox

The Firefox sync hasn't been known to cause any issues like this one.
Please try disabling it anyway.

Okay. As soon as I find out whether the next add-on is doing it.

Wait... how do I disable it?

Tools > Options -> Sync = in the lower-left corner Unlink This Device will disable Sync on that device

If those mysteriously saved bookmarks aren't appearing on the other devices you have connected to that same Sync account, I doubt if Sync would be at fault.

Two extensions that you have installed and are currently inactive might have installed additional software, and might not be totally disabled.
Default Manager 2.2 (Inactive)
SweetPacks A8 10.20.3.520 (Inactive)

Okay, I've confirmed that sync isn't causing the problem, although the mysterious bookmarks are appearing on my other computer as well.

I've returned to testing the various add-ons.

I have a similar issue - but it only started yesterday.

I've been noticing Firefox randomly add, and occasionally delete, bookmarks. Most often this happens to the "bookmarks toolbar" but I've been seeing random new bookmarks as well.

I have Sync enabled between 2 PCs, and have for months now, never with any issues.

This morning when I turned both PCs on, I discovered I had over 100 different copies of the "bookmarks toolbar" folder in my bookmarks. Some of them with truncated names like "bookmarkstool" and "Bookmarksbar" but easily 100+ all at the bottom of the list.

I deleted all these folders, reordered my actual Bookmarks Toolbar, and restored the missing bookmarks from the Bookmarks Toolbar.

15 minutes later, the Bookmarks Toolbar is out of order again, and half a dozen "BookmarksBar" folders have reappeared.

I closed and reopened ONE of the PCs during this time, but not the other.

I also just noticed that my "Unsorted Bookmarks" folder contains multiple copies of many (though not all) of the bookmarks that are in my "Bookmarks Toolbar."

I suspect Sync is somehow screwing up, and loading backup copies as it syncs or something similar?

I'm going to try turning it off for the time being and see if that helps... though it's worked fine in the past.

One more development: before turning off Sync, I attempted to get everything correct on one computer, and then "overwrite" the sync data with what was on the correct machine through the "reset sync data" option.

I did this. Reloaded FF on the second machine, and I watched to Bookmarks Toolbar reset itself properly.

Then 1 min later, both the second PC and the first (the one that I'd used as the good one to overwrite the sync data) LOST all the bookmarks in the Bookmarks Toolbar. They now match, but have no bookmarks in them, except for the subfolders I created there.

A quick check shows I now also have several dozen copies of "recent bookmarks" and the "bookmarks toolbar" folders in my bookmarks.

I've attached a screenshot of this nonsense.

Hi Panamon, assuming Sync is causing this problem, I trust you have unlinked it by now. If not, please do.

Firefox creates daily backups (unless nothing has changed) of your bookmarks, and I think you will need to restore the last backup to get your bookmarks back. Please see the following article for how to do that: Restore bookmarks from backup or move them to another computer.

I actually had a weird bookmark issue early yesterday with things getting deleted (I didn't think too much of it at the time - I assumed I was accidentally hitting some keys or something, though now I suspect it's part of the same problem somehow).

So I figured out how to restore the auto backups then, and made a local backup in case anything happened.

I decided to disable Sync on one PC and leave it running on the other. With only 1 device connected I don't see that it could screw anything up, but I figure it makes for a decent test case anyway.

Hi Panamon, unfortunately, the database file which stores bookmarks and history can occasionally become corrupted. The following post by cor-el has links to some resources on diagnosing that: https://support.mozilla.org/en-US/questions/973086#answer-485789

Okay, at this point I've gone through every Add-On except one, and disabling any of them did not stop the problem.

So now it looks as if the problem is either caused by Adblock Plus, or not caused by any one Add-On at all.

According to Places Maintenance, the file is both "sane" and "coherent", which I assume means that it is not corrupted

You can still consider to rename or move the places.sqlite file temporarily to see if it has effect.
Then you can undo this in case it didn't help.

Clearly there a serious issue with Firefox.
Please try completely removing Firefox for your system. ^{(Full Steps)}
After fully removing Firefox, please try reinstalling it from here.

Please tell us if this solves your problem.

Okay, I've just confirmed that Adblock isn't the problem, so I'll try your suggestion.

What'll I loose if I do this, btw?

-deleted by owner-

Personal attack removed by a moderator. Please note that most of the people you see here are volunteering their time to help you; as such, please treat these volunteers with respect. If you respect these volunteers, you are likely to receive an answer - Rules & Guidelines.

As I mentioned previously, I've had Sync (the built in one now included with FF) disabled for some time. I haven't had the issue since. I assume it's the culprit. BUT I'd really like to be able to use it, so I'm hoping another update comes out that addresses whatever happened to Synch around version 25.