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 box. 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 box.

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 and favicons.sqlite files contains your History and Bookmarks. And it looks like they are corrupted.

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

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 files. Then rename or delete them. 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.

Thank you for sharing your knowledge. However, prior to getting your solution, I followed FireFox's advice to Restore (Bookmarks/Manage/Import-Backup/Restore with yesterday's date). I own 2 laptops and of course, I hit the sync button on the laptop which had the loss only to find that I caused it to "spread". I did the Restore on that one and then hit sync. Do I need to complete any of your suggested tasks at this point? My profile folder under Application Basics shows my bookmarksbackups with today's date. Am I all set? Thank you

Have the missing bookmarks been restored? If so you are all set.

All Bookmarks were restored to the selected date. Phew! Your solution provided to this non-techie user was way over my head and seemed overly complicated/too much (maybe a little too deep). The easier solution on FireFox Support seemed less frightening and it also solved the problem. It is good to know that this forum has highly knowledgeable support such as yourself when a supposed simple fix could turn out to be no fix at all. Thank you

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution.

Thank you for contacting Mozilla Support.