Maybe it sounds silly but make sure you have access to the email associated with your Mozilla account. I learnt the hard way after reinstalling Windows that Mozilla sends a confirmation email to this address when you set up Sync on a new system. It turned out that I had no access to this email anymore since the provider went out of business. I was told it's impossible to access my sync data. Thankfully I did not use it to store any passwords (I use KeePass for that) because otherwise it would have been a catastrophe.