Before anyone asks, This is the Java version I have (straight from "java -version"):
java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) 64-Bit Server VM (build 20.1-b02, mixed mode)
When I installed the plugin for my x64 Java to Nightly all seemed to go well. I restarted Firefox and opened up the test page for Java... during which the browser crashed without any warning nor error message. When I reopened the browser it started without even hinting that it had crashed just 20 seconds before. I followed the steps on this troubleshooting guide and found that it was indeed the Java plugin that was causing the crash so I followed directions on the same page to uninstall the plugin. When I went back to a page with a Java applet in it, the browser correctly asked me to install the missing plugin, but when I did it said that there was an error doing so, stating the plugin was already installed.
I'm at a loss for what to do next because Firefox does not say it has any plugin for Java installed yet the error when I try to install the plugin states that it is installed.