It is possible a registry cleaner like ccleaner will do the job. It depends on how good a job the cleaner does. I'm not surprised uninstalling Firefox completely and reinstalling did not fix it. I tried that as well. The issue isn't directly with Firefox but with Java itself, or more specifically the Java uninstaller leaving things behind.

I just had the same problem of only having the Java Deployment Toolkit Plugin but not the Java Platform plugin. This was because I had installed the Java 7 JDK, which also comes with Java SE. So I thought I was good. I took a chance and downloaded and installed the Java SE as well and the Java Platform Plugin showed up in the Firefox Plugins list. Maybe some of you with this problem have just downloaded the JDK too.

alandadams, Thanks for reporting that you were able to solve the problem with the Java platform plugin not being detected. You said you had installed the Java 7 JDK and what solved the problem was downloading and installing the Java SE... by which I'm assuming you meant the Java JRE. Both the Java JRE and JDK 7 can be downloaded from the Java SE Downloads page, which says:
Quote:

"What Java Do I Need?" You must have a copy of the JRE (Java Runtime Environment) on your system to run Java applications and applets. To develop Java applications and applets, you need the JDK (Java Development Kit), which includes the JRE.

One quick question, the Java 7 SDK you had installed was also the 32-bit (Windows X86) version?

It's puzzling, since the JDK (Java Development Kit) is supposed to include the JRE according to the paragraph I quoted above and http://www.oracle.com/technetwork/java/javase/jdk-7-readme-429198.html#contents ... maybe there was an option during JDK installation to include the JRE that Firefox and other browsers need? (See http://docs.oracle.com/javase/7/docs/webnotes/install/windows/jdk-installation-windows.html#private "Private Versus Public JRE".)

In any case, thanks for the information. This thread is already over 5 months old, so if you want to reply, please do it quickly, since threads are closed and locked automatically at 6 months!

As you noted, the JDK does come with a JRE. When I installed the JDK I did check and see that the JRE was installed. However I did not have any browser plugins.

I did not uninstall the JDK when I downloaded and installed Java SE from Oracle's web site. I actually need the JDK as well as the JRE.

My guess one of the reasons why it is called Java SE is that it contains the JRE and all of the browser plugins. (Along with other things like antivirus tools and/or browser toolbars from other vendors that you have to "unckeck" to not have installed.)

I installed the Windows 64-bit version of Java 7. I doubt there is a difference in this behavior with the 32-bit version.

Firefox is a 32 bit application and thus requires the 32 bit Java version.
Firefox won't work with 64 bit Java, so you need to install at least the 32 bit JRE version.

Since my Firefox and other 32 bit Java apps seem to be working just fine, I hunted around my file system and found that while I only have a 64-bit JDK. I have both 64 and 32-bit versions of the v7 JRE. I had neither before my Java 7 JDK and Java 7 "SE" download/install. (I cleaned out my Java directories first)

It appears that some smart person at Oracle figured out that most Windows folks installing the 64-bit JRE would need the 32-bit JRE too and included it in the install package.