I forgot my Firebird server password, how to get it back?
You can't. Passwords are encrypted using a one-way encryption algorithm and cannot be retrieved. When you log in, the server encrypts the password you enter and compares that hash with what is stored in security database.
So, the only way is to reset the password, i.e. set a new one. If you still have SYSDBA password, it's easy as SYSDBA can alter password of any user. If not, you need to replace the security.fdb with a clean one (where you know the password).