I often hear these two words used interchangeably.

Problem:
These two words, whether as adjectives or as adverbs, are not quite synonyms.

Explanation:
Although it is true that “farther” and “further” have been used interchangeably for years, grammarians today increasingly consider these two words to have a physical-versus-metaphorical distinction.

As an adjective, the primary meaning of the word “farther” is more remote or distant than some place or something closer.

An example of the use of “farther” as an adjective is “The truck is on the farther side of the warehouse.”

As an adverb, the primary meaning of the word “farther” is to or at a great distance.

An example of the use of “farther” as an adverb is “Chuck Yeager flew farther toward outer space than did any of his contemporaries.”

As an adjective, the primary non-“farther” meaning of the word “further” is more extended.

An example of the use of “further” as an adjective is “Any further delay in our flight home would mean that we would not be home for Christmas.”

As an adverb, the primary non-“farther” meaning of the word “further” is to or at a more advanced point.

An example of the use of “further” as an adverb is “I may not go further with my studies until I pass that exam.”

Solution:
Use “farther” only for physical distance; prefer “further” for metaphorical distance (in space or time).