Because it is not supposed to be in the plugin directory. From Mozilla.
Java Runtime Environment
1. Install Java Runtime Environment.
2. Make a symbolic link to libjavaplugin_oji.so in your Mozilla Plugins directory. Unless you are using an old version of Mozilla, or one you compiled yourself with gcc 2.9x, use the copy located in the plugin/i386/ns7 directory of JRE 5.0, or plugin/i386/ns610-gcc32 if you are using JRE 1.4.2.
"Do not copy the plugin to your plugs directory. If you do, Mozilla will crash any time you attempt to view a page containing a Java applet. " http://plugindoc.mozdev.org/linux.html
"On Linux, Mozilla requires JRE 1.4.2 or later.
Mozilla 1.4 and later, and Mozilla Firefox, are compiled with gcc 3.x. A gcc 3.x compatible version of the Java plugin must be used. JRE 1.4.2 and later contain a compatible plugin.
If you installed the JRE 5.0 RPM, this plugin is /usr/java/j2re1.5.0/plugin/i386/ns7/libjavaplugin_oji.so - and to install it for Mozilla (including Mozilla Firefox), do the following:
* Open a terminal
* Change to your Mozilla (or Mozilla Firefox) plugins directory
* Issue the following command: ln -s /usr/java/j2re1.5.0/plugin/i386/ns7/libjavaplugin_oji.so
Important! If you install your JRE in a different way (self extracting package, debian package, etc), libjavaplugin_oji.so will quite likely be in a different location. Do not just blindly use the command listed above!
Note: In JRE 1.4.2, this file was in plugin/i386/ns610-gcc32
Important! Always make a symbolic link, as shown above, instead of copying the plugin. If you copy the plugin, your browser will crash every time you open a page containing a Java applet. You have been warned!
If you are using an older Linux distribution, you may need to install the gcc3 support libraries, as the gcc 3.2 version of the Java plugin requires libgcc_s.so.1 to operate. You may be able to find packages using Google.
If you are using an old or unofficial build of Mozilla (1.4a or later) or Mozilla Firefox, you can check which compiler was used by entering about
:buildconfig in the location bar and pressing enter. You will see a line such as "gcc version 3.3.2", which will show the compiler that was used. If gcc2.9x was used, you need to use the ns7-gcc29 or ns610 plugin, not the ns7 or ns610-gcc32 plugin."