One of the things I really liked about Firebird the last time I played with it is that it could be run over a socket or embedded as a library. The PostgreSQL community will not provide a library engine because of the fear that application bugs may cause corrupted data structures in the library. Again this is wholely consistent with PostgreSQL not really caring about the single app space that much.
What this means is that if you want to distribute an application, you can actually directly link your application to the database management component directly and this simplifies the management of your app. If you need to later move the app up to a dedicated server you can do that. Firebird makes this really easy and it does so without creating the flexibility on input (and hence the fact that so many variations must be handled on output) that MySQL does.