The latest updates to CentOS 5.5 seem to have broken the Java plugin, and have defeated any and all attempts to get it working again. I'm running CentOS 5.5 (32-bit) and Firefox 3.6.9 (installed from the CentOS repository); I've tried BOTH the openJDK plugin available through the Argeo repositories, and installing Java 1.6.0 directly from Sun/Oracle and creating the plugin soft link in /usr/lib/mozilla/plugins. Neither works at all. This was working a while ago, but it broke and I didn't notice.
Starting in Firefox 3.6.7 (I think) the (Sun) Java plugin changed names. Here is what it should look like ...
$ ls -la /usr/lib/mozilla/plugins/libnpjp2.so lrwxrwxrwx 1 root root 37 Jun 24 19:32 /usr/lib/mozilla/plugins/libnpjp2.so -> /usr/java/latest/lib/i386/libnpjp2.so
The plugin name changed .. the old one plugin is still in the jdk, but will silently fail in the browser.
Hope this helps.
Barry