You don't have to use this addon to disable the migration wizard on the initial startup. You only have to put "override.ini" to the application directory.

For Firefox:

Put "override.ini" into installation directory inside browser directory.

e.g.) In Windows: C:\Program Files (x86)\Mozilla Firefox\browser\override.ini

For Thunderbird:

Put "override.ini" into installation directory.

e.g.) In Windows: C:\Program Files (x86)\Mozilla Thunderbird\override.ini