How to get firefox to see MS java?

Hi,

After having installed XP and applied service pack 1/2 there is some sort of java present - not sure if its sun or microsoft.
But if i use IE to go to a website requiring java it works - but if i use Portable firefox it says i must download a java plugin.

Isn't there some (easy) way to get it to see the already working java on the system?

(I prefer NOT to install suns java, too big and somewhat unnecessary since there already is a java on the system)