What is the difference between Interbase and Firebird? I'm interested in a broad overview of how they are diverging and why.
InterBase version 6 was released under an open-source license in mid-2000, and was immediately forked to become the Firebird project.

Later versions of InterBase reverted to closed-source and the projects have diverged. Both are still actively developed. Developer’s tools and libraries traditionally support both servers but they are growing apart.