If you’re expecting some sensation, I’ll not make you happy. Shortly, yes, Firebird is free, completely. You can use it wherever you want and you don’t have to pay anything nor release sources of your application nor …
On the other hand, the whole truth, considering all the edge cases and consequences, is different. To keep high quality of final product, keep adding new features, provide bug fixes - simply moving forward - the creator needs some resources. If these resources will not be available, it will be effectively dead. The resources I’m here talking about, in case of Firebird, are people doing full time or regular development. These people have families, houses, hobbies, … And for all of these items you need money (in our society :)). They’re not doing it for fun (only), but also for living as well.
So Firebird actually needs some money to keep moving. It doesn’t have licenses to buy or something like that. We’re simply relying on the fact, that people using it, similar to people working on it, do love it. And are educated enough to realize all this and provide, even small, support. Thus next time you’ll be deploying your application with Firebird, think about sending $10 or even $1. I bet it’s nothing for you (compared to price of the application or money you’re paying for toilet paper in your office). And ten thousand people (not much) donating $10 makes a huge difference. It’s not only about few donating $10000.
And by the way, Firebird is not “just” engine, but tools around too: .NET driver ;), Java driver, documentation, QA, …, you name it.