Firebird 2.5 package is now replaced by Firebird3.0 in Debian Unstable.
And here is the reason from Damyan :
Please remove the firebird2.5 source package and the corresponding
binary packages from unstable.
According to the roadmap¹, upstream intents to drop support for
Firebird 2.5 in 2017.
Debian has the next major version 3.0 in sid/stretch, so I want to
avoid the need to support for stretch two variants of the codebase,
one of them possibly without upstream supprt.