Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Places Maintenance {web link} Allows to run Maintenance tasks on the database that drives Places, the bookmarks and history module behind Firefox.

Open the Add-ons Manager. Locate the add-on and press the Options button. Then follow the directions.

First suggestion - no response, problem still present.

Second suggestion - no response.

This is on my husband's Mac computer - so I run up and down stairs to attempt a fix. Fortunately he also has Safari browser so is able to use the computer. But the vast majority of his bookmarks are on Firefox. He's taking it in to be looked at tomorrow. If they can't fix it, he will have it totally removed from the computer - and will add Chrome as an alternate.

Thanks for responding.

Firefox has an auto-backup for bookmarks, but we need to fix your current problem first.

Let’s do a full clean re-install; Download Firefox For All languages And Systems {web link} Save the file. Then Close Firefox.

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder.
https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac Linux: Check your user manual.

Do Not remove the profiles folder.

Look for, and rename any Mozilla or Firefox program folders by adding .old to them. Do not rename the Mozilla Thunderbird folder.

After rebooting the computer, run a registry scanner. Then run the installer. If all goes well, remove the OLD folders when you are done.

If there is a problem, start your Computer in safe mode and try again.

Starting The Computer In Safe Mode;
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