There are apps / websites (e.g. in banking) that use one-time-passwords (i.e. different every time), but require the same username. Also, many times this username is a "random" combination of letters and/or digits, so hard to remember. Currently it is not possible to make Firefox remember the username without a non-empty password. The solution I use is a single dummy character as the password, which I need to delete every time to enter the OTP. Thanks!