Hi jayalison9, I'm sorry to hear about this problem.

Do you get a specific error message from Firefox that would help shed light on the cause of the problem?

Do you still have physical access to your installation of Firefox 66, or more specifically, to your profile folder containing your bookmarks, logins, and other data?

More info about the profile folder (hidden under Application Support):

• Profiles - Where Firefox stores your bookmarks, passwords and other user data
• Recovering important data from an old profile

On Mac you may have Time Machine backups available that you can check.

• ~/Library/Application Support/Firefox/Profiles/<profile>/

In Mac OS X v10.7 and later, the "~/Library" folder in the Home directory is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders#Mac_OS_X

jayalison9 said

... but it won't sync the data held on 66.03

When Firefox connects to Sync (on both new and existing devices) it sends data to and receives data from the Sync server, there is no direct connection to other existing devices connected to the Sync account / Firefox Account.