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.

http://www.firebirdsql.org/en/roadmap/

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.