> Should I restrict Jboss to to <track-connection-by-tx/>?

Yes. Firebird transactions are bound to the connection, so this setting is
very important.

> Should I use the JDBC driver directly instead of the JCA adapter?

No, I don't think so. However, you might want to upgrade to Jaybird 2.0.1,
some bugs were fixed since 1.5.6.

Roman