Have you tried apt-get update?

My guess, nothing more, is that you should consider pointing your sources list at unstable. In a sense, you're asking for trouble if you use a mixed sources list on a fairly old version of Knoppix....... especially with something as freaky and complex as Mozilla.

As to the "imminent failure" message, take it seriously. False warnings are relatively uncommon. It looks like it will die. Soon. Buy another. Now.