Use of the words farther and further

According to the Little Oxford English dictionary, the use of the two words are interchangeable. To be specific,

Farther refers to length or distance. It is the comparative form of the word far when referring to distance.

Further means "to a greater degree," "additional," or "additionally." It refers to time or amount. It is the comparative form of the word far when meaning "much."

Hence, it is correct to say 'The frog can jump far but the kangaroo can jump farther.'.