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 -> Show All 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, but the reply information is a bit "over my head." I've had to do a bunch of "searches" to try to decipher it so that I can understand how to proceed.

Even the simplest mention of "Sync" was foreign to me. I now know that refers to phones, pads, etc., beyond laptop. So, I don't use Sync.

It took some searching to understand how to find, "(about:support) page.

I scrolled down & found "Places Database" & clicked on "Verify Integrity." I didn't understand the list of information that appeared*, but I closed out of Firefox & restarted the computer.

- Unable to check favicons.sqlite integrity: Error: Error(s) encountered during statement execution: > Task: invalidateCaches - The task queue was cleared by an error in another task. > Task: checkCoherence - The task queue was cleared by an error in another task. > Task: expire - The task queue was cleared by an error in another task. > Task: originFrecencyStats - The task queue was cleared by an error in another task. > Task: vacuum - The task queue was cleared by an error in another task. > Task: stats - The task queue was cleared by an error in another task. > Task: _refreshUI - The task queue was cleared by an error in another task.

I was able to use "bookmark current tab" twice successfully & then it stopped working again.

I did more searches for how to find "Profiles" & I see "favicons.sqlite" & "places.sqlite" but I don't know how to tell if they're repaired.

At this point I've stopped.

The presence of "The task queue was cleared by an error in another task." is an indication that there is a problem with places.sqlite like I wrote above.

You will have to rename (or delete) places.sqlite to force Firefox to create a new file. See my above reply for more detail.