issue the following on your shell as root:
this should display a license agreement for the JRE, then you have to respond as yes! or no if u don't like the agreement
this unpacks a file, the actual RPM file, to the same directory, as i recall with the following filename: j2re-1_4_1_04-linux-i586.rpm
issue the following as root, just check the exact filename:
rpm -ivh j2re-1_4_1_04-linux-i586.rpm
this installs JRE!
in order to use the java plugin in firebird issue the following:
ln -s /usr/java/j2re-1_4_1_04/plugin/i386/ns610/libjavaplugin_oji.so /path/to/firebird/plugins/
P.S: i'm really not sure about the exact filenames or the path names, since it differs a bit in each release of JRE!