Continuing on what appears to be a string of "troubleshooting" posts, here is another one.
At work I need to use a web-based, Java-based, application to manage some of our biggest phone systems. Generally I go ahead and use Internet Explorer 7 for this particular web-site Java launcher.
But I did run into a funky issue.
On my systems I have installed Sun's Java SE 6u10-b28 build version which is now available for download. This "beta" 10-b build run promises to bring Next-Generation Java Plug-in Technology to Firefox and Opera. More here.
This version runs the particular Java app just fine in IE7. But when I tried to run it in Firefox, I got a prompt to install Java 2 Platform, Standard Edition (J2SE) 1.4.2 which is "required" by the vendor as the official supported version for it's management too.
Whatever. Didn't seem to be an issue for IE7 but Firefox is a bit different.
So I downloaded it anyway and installed it. Rebooted and tried again in Firefox.
It again prompted me to install the plugin.
I checked my Firefox Plugin list and there it was, along side the other newer Java builds as well.
Finally, after much more troubleshooting and Googling I found a fix from poster "dpinol".
Java Plug-In - java plugin 1.5 not working with firefox or netscape - Sun Developer Forum
I have the problem on WinXP sp2 with Firefox 1.0.3 and JRE 188.8.131.52
Probably because I've installing and uninstalling several browsers and JREs.
I still cannot check the applet checkbox at the control panel, but now applets work.
To solve it, I copied all these files
from C:\Program Files\Java\jre1.5.0_02\bin
to c:\Program Files\Mozilla Firefox\plugins
I did that (making the necessary location changes for my particular Java build and system particulars) and restarted Firefox one more time.
Now the Java application launched without issue from the hyperlink.
Curiously, my Java 6 build program file had these same files in it's folder, but the Java SE 6u10-b28 program folder did not.
Though someone might find this helpful if they are having backwards-compatibility or Java Plugin issues like I did.