Sun Java Plugin 6 does not work in Firefox on F7

[Marcelo Magno T. Sales]

People,

The Sun Java plugin which comes with JRE 6 does not work correctly in 
Firefox on several F7 machines I have here. It shows only a blank 
space at the place where an applet should be in web pages. No errors, 
no messages, but no applets :) There are a few F7 machines, however, 
where the plugin works fine, but I can't detect any difference 
between them and those where the plugin does not work.
The plugin from JRE 5 works fine on all machines.
Why is that? How can I make the plugin from JRE 6 to run consistently 
in Firefox on F7?

Thanks,

Marcelo