Well, the plugin itself (which comes from the portion of Oracle that once was Sun) couldn't be gone -- Firefox can't delete chunks of my JRE. Only the link to it was gone. There is a handy Synaptic meta-package (sun-java6-plugin) in one of the Ubuntu repositories, intended (I think) to make the connection between plugin and browser. I reinstalled it, to no avail. I'll skip the full litany of web search results (many out of date) and failed attempts, and cut to the chase scene. Besides having a recent Oracle/Sun Java JRE installed (and, of course, Firefox), you need to do something to tell Firefox where to find the Java browser plugin. The rest of this is specific to Linux; Windows and Mac users will have a somewhat different task.
- You need to find a Firefox plugins directory, in which to place a symbolic link to the plugin. There are multiple choices, and more than one of them may be suitable. Some documents refer to a plugins folder under ~/.mozilla (although I suspect this now should be ~/.mozilla/firefox/<your configuration>). I didn't have such a folder, so I created one (although I ended up not using it). Another possibility is /usr/lib/firefox/plugins. I'm not sure if that one is scanned, or if it's just vestigial from some earlier version. I ended up using /usr/lib/firefox-10.0.1/plugins ... which I may regret if I have to go through this again with the next upgrade.
- You need to find the right plugin file. This is where I wasted most of my time. Older documentation refers to libjavaplugin_oji.so, which I found in /usr/lib/jvm/java-6-sun/jre/plugin/i386/ns7. It's in a folder named "plugin", so it must be the right file, mustn't it? Oops, no. It turns out there's a "next generation" plugin named libnpjp2.so, and apparently FF 10 will only recognize the new plugin. Oracle's installation docs refer to it, but I could not find it in the plugin directory. That's because it's not in the plugin directory. I guess they thought that would be too obvious. It's in /usr/lib/jvm/java-6-sun/jre/lib/i386.
- Once the targets are in sight, shut down Firefox, go to the directory you selected in step 1 (in a terminal), delete any symlinks to older plugins, then execute sudo ln -s /usr/lib/jvm/java-6-sun/jre/lib/i386libnpjp2.so, adjusting the path as needed if your JVM is installed elsewhere. (The sudo is unnecessary if you chose a folder in your home partition in step 1.) Restart Firefox and check the plugins tab (or enter about:plugins in the URL bar) and make sure Firefox acknowledges the plugin.