During the day I realized that MOS was acting strangely since I upgraded to FireFox 3.6 and @rnm1978 confirmed my findings just now:
You can’t directly read MOS knowledge items with Firefox 3.6

Opening a knowledge items as usual (left-click – which should display the content on the right hand side of the screen) just pulls up a blank display:

You have to right-click the item and choose “Open Item in New Window” to see its content.

Not very comfortable but there’s always supporthtml.oracle.com as a fallback scenario.

Cheers,
Christian