Let's start with; If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. 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, check this link: 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.

You can't use Sync to sync old history items. New history is only uploaded for the last 30 days. Synced history expires in 60 days.

You may want to create a backup copy of places.sqlite since there is no backup possible for the history apart from copying history to bookmarks.

You can check for issues with the places.sqlite and favicons.sqlite databases 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.

See also:

• https://www.nirsoft.net/utils/browsing_history_view.html

Thanks to both of you.

I ran "Verify Integrity", and it didn't report any errors. Part of what it said is: > Task: expire + Database cleaned up > Task: vacuum + Initial database size is 51200 KiB + The database has been vacuumed + Final database size is 46080 KiB

When I looked at the History menu, the option "Show All History" was still grayed out (or at least the shortcut). But then I nevertheless tried hitting Cntl-Shift-H, and lo and behold it worked!

A couple more questions: From where was it getting the suggested URLs yesterday, which were URLs I had used on my new computer? I would think they would come from places.sqlite, but as you know the history wasn't working!

If I do a backup of places.sqlite, then if ever the real one gets corrupted, is it that all I have to do is overwrite the real one with the backup, and everything will work?

Any idea why history didn't work, but was able to be restored so easily?

If a file is found to be corrupted, you should either delete it or replace it. What you can do is to Copy a backup file over (overwrite) the old file.