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