Firebird may take longer to get into than SQLite, but if you believe that your application will grow, need more powerful SQL features or need to migrate to client/server mode then its well worth putting the effort in to use Firebird. Adding client/server is very straightforward. Indeed, I normally create two versions of my apps, one for personal databases and the other for client/server. The difference between the two is very minor and mostly to do with prompting the user for login credentials.
Even better, using Firebird means that you can also use the powerful IBX components
IBX also comes with examples and components for embedded use, including backup/restore, database initialisation and schema upgrade.