Which of the following databases is best for a big desktop application: Firebird, JavaDB, hsqldb ? I need performance, easy of use, and totally free license.
If you're looking for an embeddable database, my recommendation would be H2 (stands for Hypersonic 2, (re)written by the author of HSQLDB). See the detailed comparison of features and of performances.
Using a 100% Java database would allow to run it in the same process and make things a lot easier IMO.