I suspect the relevant files are places.sqlite, which stores history/bookmark URLs and related information (such as bookmark titles and history visit dates), and favicons.sqlite, which stores the site icons for history/bookmark URLs. I don't know why any other program would be using either of those files.

Maybe that is a bad guess by Firefox and the file is locked for another reason, such as Firefox not having permission to access the file. Are you using the default profile folder created in your Windows user profile, or did you copy in a profile folder from your previous computer? If you dropped in a folder from another computer, you may need to go into the security properties and reassert ownership of the folder or otherwise grant your new Windows user full permissions.

I copied the files from an old profile. I made sure the new profile was read/write. The toolbar is fine, and the page is fine. The only problem is the bookmarks. I'm not sure how to check file ownership, or who should be owning a file.

It's in the Properties dialog, Security tab, Advanced button. The issue is that even if your user name is the same on both systems, the underlying SID is different, so there may be access problems. It has been many, many years since I've had to deal with this, so you may want to search on Windows sites for advice on ruling this out.

If you use Sync, 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 -> Manage 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

jscher2000 and cor-el, thank you. I deleted all the "places.sqlite" and "favicons.sqlite" files and restarted Firefox. I was able to restore my bookmarks, and now all is well.

Thank you both.