(9 October 2014 edit...)
good advice wrote:Here's instructions on how to "delete" the Java DT plugin "for now." Note: it probably will come back when you update Java in the future:
1. Open "about:plugins" without quotes as a URL in firefox. (You might need to click "yes/OK" to a cutesy message that pops up saying something like, "changing these settings could void your Firefox warranty!" or something along those lines.)
2. In the about:plugins page, scroll down to where it says "Java Deployment Toolkit", make note of the associated .dll file (it's probably "npdeployJava1.dll" but it could be different for you), and then copy the path to the .dll file (everything before "npdeployJava1.dll".) The path can be different depending on which version of Java and OS you have installed. For me (as an example), I had a 32-bit version of Java v7 installed on Win7 x64 and that path was: "C:\Program Files (x86)\Java\jre7\bin\dtplugin"
3. Close all Firefox windows, this is important.
4. Now browse to the path location you copied and from there you can delete the .dll file that's associated with the Java DT plugin (again, it's probably "npdeployJava1.dll"). There might be more than 1 .dll file in that location but just delete your noted .dll file that was associated with Java DT.
5. Open Firefox and the Java DT plugin in your list of plugins should be gone.
Additional notes:
1. It's possible there may be more than 1 .dll file listed for the Java DT plugin(s), for example you have multiple versions of Java installed like v6 and v7 at the same time and in that case there might be a v6 DT plugin and a v7 DT plugin. I've confirmed Java v8 also has a DT plugin as well so be sure to note exactly what "about:plugins" is saying about the Java Deployment Toolkit plugin(s).
2. As stated above, the Java DT plugin will probably "come back" when you update Java. My recommendation: a more permanent solution to get rid of these plugins (and if you don't need Java content running in the browser) is to turn off Java completely for the browser and Oracle has a good, simple guide on how to do that. Following that guide will get rid of all the Java plugins immediately (so you wouldn't even need to follow the previous instructions) and will also prevent these plugins from reloading/reinstalling in the future when you update Java. This method is ultimately safer because you wouldn't need to worry about the sensationalized "drive-by" Java exploit attacks on the browser if you were to accidentally allow a malicious Java web applet to run.
3. I wouldn't be surprised if these instructions are out of date in a couple months considering how many changes are made to the Firefox config and GUI on a monthly basis. A lot of people here like to hold onto ESR versions of Firefox and legacy versions of software like Java v6 because they're resistant to change and I kinda frown on that behavior from a security standpoint. Or, it could just be that the applications you use are out of development and you simply need legacy versions of browsers/Java in order for them to work -- I've been in that situation before and understand you. However, if you're the kind of person who is resistant to change, I don't believe Firefox is the right browser for you. In case these instructions aren't working, I suggest replying to this topic (if it's still open) and make a few comments if things aren't working. I might update these instructions periodically.
(continuing my original post from 6 October...)
Anyway, after finding many mozillaZine threads (and countless more Mozilla support questions) on this plugin and also finding somewhat dated information on the Knowledge Base article, I'm interested in bringing up the discussion again due to this plugin's affinity to cause mass confusion and its ability to come back when you think you've deleted it. It will come back once you update Java. It came back for me and I was just curious if any of you experienced the same. I guess I'm looking for a way to permanently never see these plugins again, and turning off all Java content in the browser seems to be the answer for me. Sites like Pogo are already moving away from Java-based gaming and my Java applications like Minecraft will still run just fine in a standalone (non-browser) environment.
Doing a little bit of digging, I'm finding quite a bit of Mozilla drama in regards to this plugin and I question why Oracle would keep reinstalling this plugin when the user updates "Java." I'm assuming that Oracle knows that firefox is still perpetually blacklisting this plugin as "vulnerable" until security issues are resolved. Why wouldn't Oracle offer Java DT as a "niche" download if it was intended for a niche audience who are keenly aware of its security implications? Why cause the mass hysteria and bundle it with Java SE? Might be a question for Oracle and not here though.
P.S. For those of you looking for "alt fixes," I would strongly discourage things like registry hacks. Firefox configuration settings get turned on a dime every other week. Before you know it, your entire installation will probably get corrupted in an attempt to prevent firefox from auto-scanning new plugins.