Things have been taking slighty longer than expected with the first stage of the Firebird integration project, I have however finally gotten Firebird building within LibreOffice both on Linux and Windows (Mac OS still to be tested).
Firebird (2.5) is officially only supported using MSVS 7 or 8 on Windows, however LibreOffice should support 10 (2010) or 11 (2011), hence I had to do some work patching Firebird to compile within the LibreOffice build system. This now works and LibreOffice build successfully — unfortunately the sdbc driver itself now crashes on Windows, which will need fixing.
I’m now starting to look at the SDBC driver itself (which Javier Fernandez and Andres Gomez of Igalia originally started writing) as well as looking at how to have the firebird db stored within the .odb file itself (the driver currently assumes a db in a temp. directory). Whereas HSQLDB writes directly to the .odb, for Firebird — at least initially — the database file will be stored externally and extracted from / written into the .odb file as appropriate. Ideally the driver would be able to deal both with having a completely separate db file, and having the db within the .odb — it looks like this should not be particularly difficult to implement.