You may have changed the sorting order in its window. Try to select; Name.

I think that this can happen if the bookmarks are restored from Sync as that doesn't preserve the ordering.

You should be able to restore the bookmarks from an automatically created compressed .jsonlz4 backup in the bookmarkbackups folder.

• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them
• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

This could be a problem with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

• use the "Places Database" -> "Verify Integrity" button on the "Help -> More Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder with Firefox closed. Firefox will rebuild places.sqlite and restore the bookmarks from the most recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created
• you lose the history when a new places.sqlite is created

You may have to restore an older bookmarks backup in the bookmarkbackups folder via the Bookmarks Manager (Library).

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore
• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

See also:

• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional
• https://support.mozilla.org/en-US/kb/cant-add-change-or-save-bookmarks

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

If you use Sync then best is to reconnect one device at the time and wait for the initial Sync to finish before connecting another device once you have fixed the bookmarks.

But I didn't touch the order at all.....they were fine last night as far as I know and I check them this evening and they are like this.

And, they're not just out of ORDER, all the bookmarks that were in folders and SUBFOLDERS even are no longer in those and are now just in ONE big pile at the bottom of my Bookmarks menu with all the now-empty folders at the top of the list...

If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Type https://accounts.firefox.com/settings<enter> in the address bar. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

If any errors are reported: https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

The places.sqlite file contains your History and Bookmarks. And it looks like it is corrupted.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox. The browser should then create a new place.sqlite and recover your bookmarks from the backup files.

Note: Your history is not backed up and will be lost.

I didn't shut down sync as I didn't notice that it had happened and it spread to all my devices already.

I used the 'Verify' option under the 'Help/More Troublehsooting' menu re: places.sqlite and it said that it was 'sane' so I assume that means it wasn't corrupt?

The only thing I can think of that caused this is that I recently bought a Raspberry Pi....I own two laptops (one with Linux mint 20.2 MATE and one with MX Linux 21), a Windows 10 desktop as my main PC, and my Home Theatre PC also running Linux Mint 20.2 MATE.

Everything's been working fine with those, but I DID buy and set up a Raspberry Pi running TwisterOS last week and had installed Firefox ESR on that.....the only thing I can think of is that this happened when I signed into my Firefox account on the Pi...? But the Pi was brand new with a fresh install of Firefox, so why would it have screwed up my synced bookmarks?