First read this Mozillazine page
In my case the problem was that some program overwrote the registry entry HKEY_LOCAL_MACHINE/SOFTWARE/JavaSoft/Java Runtime Environment/BrowserJavaVersion with the old value 1.6.0_01. When I set it back to 1.6.0_06 everything worked fine again.
Now the addons window looks like
before it showed Java Platform SE 6 U1 instead
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