When at first you don't succeed ...
I solved the problems I was having. Here's how:
First, deploying the firbirdsql.rar file precludes deploying the JDBC database driver file, firbirdsql.jar, since the rar file includes that driver.
Second, in the jbosscmp-jdbc.xml file, I was declaring the <datasource>ContactDS</datasource> instead of <datasource>java:/ContactDS</datasource>. After doing so, and changing the resource-ref entries to be like:
In my ejb-jar.xml, I was just referring to <res-ref-name>ContactDS</res-ref-name>, but didn't realize I had to name the <jndi-name> with a "java:/" prefix.
I'm sure that's probably documented somewhere in the JBoss docs I paid for, but I actually discovered it in the free O'Reilly JBoss Workbook beta PDF. (My thanks to Bill Burke for the tip.)