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