First, you should check your sync settings and make sure it's turned on. If it's already on, you can try reinstalling Firefox. You can also remove old authorized devices from your sync account. If none of this works, you might have to manually import the passwords from your phone. I'm Chinese, so please excuse any mistakes in my English. Hope these suggestions can help you.

Thank you for your reply. I think I have found the problem.

I didn't know that there were two Firefox profiles on my computer and it had opened up a rogue profile with no passwords saved. The password problem cured itself when I restarted after a driver update. Once the computer had rebooted and Firefox was opened, there were two profiles and Firefox wanted to know which profile to use. I tried both and one of them had the passwords etc, while the other profile did not.

It seems that when the passwords went missing, it was because Firefox had opened with the rogue profile where no passwords were stored.

Why it opened the other profile and how the other profile came into being is a mystery.

The problem now is that Firefox will not allow the unwanted profile to be deleted.

Does anyone know how to get rid of the unwanted profile that seems to be locked onto something else?