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 -> 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

I'm sorry. Looking at the about:support page, I don't see these buttons. BTW, I have tried to keep my devices and browsers from syncing, to avoid too much stuff on my phone, too much personal stuff where I work, and irrelevant stuff on the other browser I've used when working from home.

Sorry for taking up your time here. I found the button, clicked it, could not tell if it had found an error, re-started, opened Firefox again. I can see some of the bookmarks in a pull-down from the application menu, but not organized, and not the ones I care about. But the library, which I kept on the left side of the screen, is still gone. I don't know how to get at the profile folder with Firefox closed. What do I do here? It took a while to look some of this stuff up.

Do you mean the bookmarks sidebar (Ctrl+B)?

• https://support.mozilla.org/en-US/kb/use-firefox-sidebar-access-bookmarks-history-synced