Sorry. We’ve locked your account. The confirmation email we sent to xxxxx@gmail.com was returned.....

attempting to login to Firefox sync I'm told the system has just locked my account - which is odd because I just logged in with the account to log this issue. Message is : "Sorry. We’ve locked your account. The confirmation email we sent to xxxxxx@gmail.com was returned and we’ve locked your account to protect your Firefox data. If this is a valid email address, let us know and we can help unlock your account." There is nothing wrong with my gmail account either - I checked gmail is not bouncing mail back sent to my address.