I registered a sync account on firefox for Macbook and synced on Macbook perfectly. But when I tried to syn on my iphone 8plus, the same account was told not existing and I was told to create a new account. Then I deleted the sync account from Macbook firefow and registered a new one (with the same email address) on my iPhone. Then the syncing on iphone was perfect. But when I tried to sync on my macbook, the same account was told not existing either.

It seems to me that the sync account on Macbook firefox and the sync account(with same email address) on iPhone firefox are in 2 isolated zones. The 2 devices can not see each other with the same sync account of email.