What is “the” Firebird package to use today? FDB? pyFirebirdSQL? kInterbasDB?

Short answer is FDB
The difference between FDB and kInterbasDB is explained here

It’s old and unmaintained , all effort is on FDB driver and it is backward compatible with kInterbasDB
Now between FDB and FirebirdSQL , from what i know pyFirebirdSQL is pure python so it can work without any other so/dll installed and it can be used for example in places where there is no Firebird client installed (think of android python script)