When I try to sign into my sync account it says that there is not such user. I do NOT want to register again, as doing so has a perverse way of creating duplicate accounts. I also have an account password file to keep track of user login data: user name, email address used, password, so this is not a matter of just forgetting the account name etc. I just updated to Firefox 29, though this could be coincidental.

I use windows 7, with latest updates & patches.