Firebird seems stable and fast enough, what it does lack is good client libraries. We looked at different database solution when rewriting our current webshop and Firebird was something we looked at, but because we're a Python shop we dropped it. I'm not saying that the current Python driver doesn't work, it does, but it doesn't seem to be updated very infrequently and it's still seems more like an InterBase driver than a Firebird driver. We went the Postgresql route, and why really, why wouldn't you?

Don't get me wrong, it's a great database and I know a company that use nothing else and won't switch if you paid them, ( They are use .Net ).