These, like farther/farthest, can be used as adverbs of place/distance:

It isn’t safe to go any further/farther in this fog..

But they can also be used in an abstract sense:

Mr. A said that these toy pistols should not be on sale.

Mr. B went further and said that no toy pistols should be sold.

Mr. C went furthest of all and said that no guns of any kind should be sold.

B.     far: restrictions on use

far in the comparative and superlative can be used quite freely:

He traveled further than we expected.

far in the positive form is used chiefly in the negative and interrogative:

How far can you see? ~ I can’t see far.

In the affirmative a long way is more usual than far, and a long way away is more usual than far away:

They sailed a long way.

He lives a long way away.

But very far away is possible, and so is so/quite/too + far and far + enough:

They walked so far that. . .

They walked too far.

We ‘ve gone far enough.

far can be used with an abstract meaning:

The new law doesn’t  go far enough.

You’ve gone too far! (You’ve been too insulting/overbearing/

insolent etc.)

far, adverb of degree, is used with comparatives or with too/so + positive forms:

She swims far better than I do.

He drinks far too much.