I'm assuming that you have a folder called "Old Firefox Data" (or something similar) on your Desktop?

As explained in the Recover important data from an old profile documentation, you need to first copy the places.sqlite file from the folder on your desktop and then copy that into your Firefox profile. This files stores your bookmarks, history and download history.

To open your profile folder, you type about:support in your Firefox address bar and press the Open folder button under the Profile Folder section if the Application Basics.

Alternatively, you can simply press with Windows key + R on your keyboard and enter %appdata%\Mozilla\Firefox\Profiles to open your Firefox profile folder.

Hope this helps.

I did this but when I went to to show all bookmarks there is nothing there. "In the Firefox profile folder, find and rename the places.sqlite file. Also rename the files places.sqlite-shm and places.sqlite-wal (if they exist). To rename a file, right-click on it and select Rename from the menu. Then add .old to the end of its name. " Should I have deleted the 'places.sqlite file' in the Profile Folder Application Basics?

It's generally recommended that you don't delete anything until you can confirm that it's working. That way you can always restore the required files in the event that something goes wrong.

The generally recommended practice is to rename the places.sqlite and associated files to places.sqlite.old because that way Firefox doesn't see the files, but the files are still there.

Then when you copy the places.sqlite file from the Old Firefox Data folder on your desktop, you won't be overwriting any files. When you add that file into your Firefox profile, you should be able to recover the bookmarks (when you restart Firefox).

The OP sent me their places.splite file from the Old Firefox Data folder on their desktop and I am able to verify that the file does not contain their bookmarks, as is usually the case. Hopefully another forum contributor will be able to assist this user further.