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 ).
Postgres is developing a lot quicker then Firebird. Also, management and replication solutions for Postgres are far more advanced. But what matters most (for me) is documentation. I can find everything I need to know right here:
I wrote an accounts receivable/payable app in Borland Delphi back in 2002. At the time Interbase was the easy choice. At that time, Interbase was transitioning to Firebird. I dropped it in as a replacement and it did its job fine. If you like MySQL, you'll miss auto incrementing Primary Key Columns that MySQL seems to have made so popular. It's not too hard to setup a trigger for sequence types, but it's not built in. (or it wasn't back then) All the tools for management were the Interbase tools. Most of my stuff is either MySQL these days or Postgres when I need good spatial support. I've messed with CouchDB a bit. I just don't see Firebird going anywhere for my needs.
