IMO, your problems are with corrupt data within Sync and the Sync server which would continue to propagate the corruption back to the "connected" devices even with a new Profile. Also, if you have a Firefox for ... either iOS or Android version running on the Sync account that may be the initial source of the corruption; there has been an increasing incident of reports over the last (almost) 5 years with those 2 versions being the culprit per the questions posted in this fora.

To clear corruption on the Sync server you need to create a new (FxA) Firefox Account password; that will clear all your data from the server, and then be able to receive a "clean data" upload from the first device that is connected with the new password. So you need to have al least one desktop / laptop device with UN-corrupted data for the first Sync event with a new FxA password.

By opening Help > Troubleshooting Information and scroll down to Places Database and use the Verify Integrity button to run a 'test verification' and repair process for Places on that local device. Read the results and if all are deemed Sane you should be good to go for re-connecting to the Sync server with the new password.

Unfortunately, the Firefox for ... builds don't have that "Verify Integrity" feature for Places.

Awesome! Thank you for this guidance. I do indeed have an iOS device involved, so that's a great tip. I'll report back on how it goes.

These steps should normally be sufficient to recover from a corrupted places.sqlite with as a last resort deleting all places.sqlite files to make Firefox rebuild the bookmarks from a recent backup in the bookmarkbackups folder. I think that on Mac native

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

You can check for problems with the places.sqlite and favicons.sqlite files in the Firefox profile folder.

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

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

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a 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

See also:

• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

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

• Help -> 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

Thanks, cor-el, for this suggestion. I can confirm that this is a recurrent problem and that the Verify Integrity procedure, including deletion of the sqlite files, has worked only about half of the time and has not been a lasting fix—the problem recurs within 2-3 days.

I've tried this with Sync both on and off, but I've always reenabled Sync after the fix. It does seem like Sync may be a part of the problem, so now that I've deactivated Sync permanently, if the corruption recurs, I'll report back if the Verify Integrity procedure is a lasting fix now.

Thanks for both your suggestions. After a week, turning off Firefox Sync seems to have solved the problem.

My recommendation for other users troubleshooting this or similar issues is to try turning off Firefox Sync first, which – given other people's corruption, lost bookmark, and data loss issues I've read in the support forums – should probably be considered a beta product.