By now maybe you've found the answer, but just in case, here's the little memo I wrote myself after needing to install jre on firebird about 5 times:
Download the rpm and install it. It landed in /usr/java.
Firebird lives in /usr/local/firebird on this machine.
The link has to be made in the plugins folder under firebird.
Below, I go to the /usr/local/firebird/plugins directory first, and get the exact name of the libjavaplugins that exist in /usr/java with the “find” command:
root@tooty plugins]# find /usr/java -name "libjavaplugin*"
I know from past experience (and a document I can't find right now) that I need the nx610-gcc32 version of the plugin. This command makes the link. The period at the end of the command is important.
[root@tooty plugins]# ln -s /usr/java/j2re1.4.2_01/plugin/i386/ns610-gcc32/libjavaplugin_oji.so .
[root@tooty plugins]# ls
I hope this helps!