This isn't really an answer, but some things you might like to consider before making your mind up.
What do you mean by moderate-to-large scale? Do you mean by the number of concurrent users or by the amount of data being stored?
Are you going to be supporting the database as well when it's deployed, or might the customer have their own DBAs and want to incorporate it into their infrastructure team's remit?
Do you need to build-in complex features like auditing or granular security that some databases might provide out-of-the-box? Do you need to be able to run an overnight batch process on the database in some controlled manner?
Do you need to be able to have robust high-volume transactional support, or is it more for just using as a sink for reporting data? i.e. is it more OLTP or OLAP?
Are you limited by cost? i.e. does it need to be free/open-source, or can you select paid-for vendor product?
Both Firebird & Interbase are great, but not generally well-received into an "enterprise" environment in my experience unless they're "hands-off" and embedded into the application. MS SQL Server is a "safe-bet" for most systems. There is a cost associated with it but, as you know, it does come with a complete toolset and very good support.
If you do go down the Firebird or Interbase route then do take a look at Jason Wharton's IBObjects library which provides a high-performance replacement for BDE when using these databases. http://www.ibobjects.com/