Does sync store a saved password for account.firefox.com? Check your password manager on one of the devices that has the incorrect name and password.

If you clear cookies and history on close, sync will be logged out in my experience.

mrfixitnv said

I have two laptops with Ubuntu 14.4 LTS and one with Win7. All with Firefox, Adblock, Noscript and HTTPS everywhere. All three laptops do the same thing. Have privacy set to clear history with passwords under data un-checked. Sync works between devices, passwords are stored. Get the SYNC failed to connect, incorrect password or username when Firefox starts up. Firefox and OS's are up to date.

Follow-up on my post. Sync started working. Not sure why, it was after excluding passwords from being cleared at close of Firefox and after I logged into Sync from a second laptop and had it Sync.

sorry not answer, i'm looking for help myself

The Sync version used in Firefox 34 and later uses the password manager to store the name (e-mail) and password to be used to connect to the Sync server. The chrome://FirefoxAccounts entry in the password manager stores the authentication data (kA and kB) in JSON format.

Make sure the you do not use "Clear history when Firefox closes" to clear the passwords.