>>If any other application that uses Firebird is using its own
>>usernames and passwords they won't be able to create them without
>>knowing the SYSDBA password!
>
>
> So equally, if the user of my app has firebird installed previoulsy,
> and has changed the defualt password before installing my app, my
> app will not be able to create its 'own' user? Or do anything?

There can be several Firebird servers on one PC running simultaneously.
So just install your application such that it uses its own FB server.

Ivan