I used '13_13_13_13_13_!!!'

Basically, it needs to be a decent password that you will NEVER forget because I'm not sure there's a way to get all your passwords if you forget the Master password.

However, for my browsers on the PC the Add-on seems to remember that Master Password and I never have to re-enter it. And that's good and bad. Bad because if I don't use the Master Password I might forget it.