A further, furthest
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 travelled 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
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.
Fonte: A. J. Thomson, A. V. Martinet, A Practical English Grammar, Fourth edition, Oxford University Press