The "username" for a Firefox Account is the email address that was used to create that account. If you aren't getting an email it might have been blocked by a spam filter in your email client or your email hosts software, which is a fairly common occurrence; or you might be using an email service that is blocking email from the Sync server / Firefox Account server. But if you don't know the password all you can do is to reset the password, resulting in the loss of data that might be on the Sync server. Bottom line is that Mozilla doesn't have your password for your Firefox Account, it is only saved on the local PC and never is sent anywhere. You should be able to see that password in the Login Manager under https://accounts.firefox.com if you ever logged into your Firefox Account on the web. If you never logged in thru that web interface you would only see a "username" for Sync with the URL being chrome://FirefoxAccounts(Firefox Accounts credentials) in the Login Manager which would show '''{"version":1,"accountData":''' and the dual kA / kB "passwords" that Sync uses inside Firefox to connect to the Sync server - which started with Firefox 29, but that can't be used manually to sign into Sync.