I remember using it a bit in my Delphi days ... (nostalgic face) It just worked (for smallish projects) and was quite nice, though nothing ground-breaking.

> How is it that it's so little known among developers?

Both Interbase and, by extension, Firebird were part of the Borland's and, later, Embarcadero's product ecosystem (especially, Delphi and C++ Builder) and, thus, closely tied to relevant developer community, as chadcmulligan mentioned. As the overall ecosystem declined, so did the popularity and use of Interbase and Firebird.