I have properly installed a symbolic link to the amd64/libnpjp2.so java plugin file in ~/.mozilla/plugins but Firefox 3.5.9, 3.6.2, and 3.6.3 refuse to see it in about:plugins (Ubuntu 9.04, 64 bit, manual install of Firefox) - the same directory contains the 64 bit Flash plugin (which I had to update to make it see) and the libnullplugin.so. These are both picked up, my symbolic links to /usr/lib/totem/gstreamer/*.so files are not, but I presume they don't follow the normal architecture. I have the tar.gz version of all 3 versions of Firefox previously listed installed, I simply change a symbolic link which /usr/bin/firefox points to when I want to try a different version. My Firefox profile is from an older, 3.x version that Ubuntu 9.04 came with, however backing this up and deleting it, then letting Firefox create a new profile does not seem to fix the problem (i.e. Firefox still can't see the Java plugin). Note that it's easier to maintain a manual Java install on my machine since I have to develop with various versions of Java. I noted that java.java_plugin_library_name in about:config had an improper value for the old plugin name, I did switch it to libnpjp2. == This happened == Every time Firefox opened == I upgraded to a manual install of Firefox 3.6.3