FULL PRODUCT VERSION :
java version "1.6.0_13"
Java(TM) SE Runtime Environment (build 1.6.0_13-b03)
Java HotSpot(TM) Client VM (build 11.3-b02, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Windows Vista Ultimate 64-bit
Service Pack 1
Version 6.0 (Build 6001: Service Pack 1)


EXTRA RELEVANT SYSTEM CONFIGURATION :
I am experiencing this problem under a 64-bit version of Windows
I used a fresh copy of the offline installer from Sun's "java.com" web site; filename: jre-6u13-windows-i586-p-s.exe
The browser version was Firefox 3.0.8 as downloaded from getfirefox.com. Note that this is a 32-bit program running under a 64-bit OS; the Mozilla Foundation does not make a 64-bit version of Firefox available

A DESCRIPTION OF THE PROBLEM :
* Install Firefox 3.0.8
* Install JRE 6u13

- www.java.com will claim that Java is installed
- The Java plugin and DLL helper will show up in Firefox' plugin list
- Java applets will NOT run

Disabling the "next-generation plugin" and restarting Firefox results in Java applets working again. Until I found that workaround, I was stuck with 6u7 for the longest time, although I've since learned that this feature wasn't added until 6u10.

As there seem to be significant enhancements to the next-generation plugin, it would be helpful to get it working in the next version. I downloaded an early build of 6u14 from http://download.java.net/jdk6/ -- the same problem persists in the 32-bit build. (As expected, the 64-bit build does not work with Firefox at all (although it works great with IE8.)

Meanwhile, I would strongly suggest that Sun provide information about this to users of 64-bit Windows versions in a prominent location. I only found the solution after digging through numerous third-party blog entries from people who have encountered the same issue.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
* Install Firefox 3.0.8
* Install JRE 6u13

- www.java.com will claim that Java is installed
- The Java plugin and DLL helper will show up in Firefox' plugin list
- Java applets will NOT run

* Disable "next-generation plugin" in Java control panel
* Restart Firefox

- Java applets will now function correctly


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Java applets should execute within Firefox even with the next-generation plugin disabled. The only information I was able to find on the subject states that it does not work in Firefox 2, but explicitly states it works in Firefox 3. This information has been removed from newer versions of the page, so here is a link from Google Cache:

http://74.125.47.132/search?q=cache:b9VrgxUVypsJ:https://jdk6.dev.java.net/plugin2/+next+generation+java+plugin+does+not+work+with+firefox&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a
ACTUAL -
Java applets did not execute. Instead, I got a blank box on any web page that included a Java applet

ERROR MESSAGES/STACK TRACES THAT OCCUR :
No error message is apparent, even in the Java console. Applets do not execute at all

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Disable "next-generation plugin"