Calvin and I noticed if you use the following build dated 30_mar in Firefox 3 or IE, on Windows XP and Vista, an applet will not load unless you specify the JPI_PLUGIN2_DEBUG system environment variable.

http://j2se.east/plugin/1.6.0_10-pit/2008-03-30/windows-i586/jre-6u10-fcs-bin-b521-windows-i586-p-30_mar_2008.exe

However, it works fine using the build dated 27_mar_2008:

http://j2se.east/deployment/jdp-scratch/deployment/1.6.0_10/pit/2008-03-27.12_21/windows-i586/jre-6u10-fcs-bin-b521-windows-i586-p-27_mar_2008.exe

I was able to reproduce this on 2 of our machines(charminar and longhorn) in our lab. Both machines have Firefox 2 and 3 installed. Calvin mentioned that this may have been related to having both Firefox 2 and 3 on the machine. So, I removed and re-install Firefox 2, 3 and the JRE on one of the machines, and then everything worked fine. If anyone wants to take a look at this, longhorn has not been touched.

I tested this on RedHat 5 as well and it worked fine.