Hi Gilbert,
Try this,
create a softlink for the jre plugin in your mozilla plugins directory.
check for the jre plugin at
usr/java/j2re1.5.0/plugin/i386/ns7/libjavaplugin_oji.so
to install the plugin do the following,
*Open a terminal
*change to ur mozilla plugins directory
*issuing the following command
ln -s /usr/java/j2re1.5.0/plugin/i386/ns7/libjavaplugin_oji.so
if it is jre 1.4.2, this libjavaplugin_oji.so is in
plugin/i386/ns610-gcc32 directory of jre.
Dont copy the libjavaplugin_oji.so to ur mozilla plugin directory, it
will crash the browser while loading a java applet.
Assumption is made that ur jre is installed in /usr/java/j2re1.5.0, if
other please change the above command as suited.
Dont forget to enable java in ur preferences of firefox.
For more information pls. refer
http://plugindoc.mozdev.org/faqs/java.html#Linux