gone Postgres for almost everything nowadays [...]
Does anyone have experience with Firebird to share?
And there you have it; you really said it yourself in a nutshell. I'd be somewhat interested in firebird too. But who is going to mess with a relative unproven option like firebird? (except perhaps for the embeddedness -- which Pg does not offer)
IMHO, even if one assumes that firebird can catch up with postgres at all, it would take quite some time before it will be able to match postgres' stability and 'ecosystem'.