The word farther is used for distances:
takes about 5 hours to get there.
You can remember it because farther has the word “far” in it – which refers to physical distance.
The word further is used for metaphorical distance – such as time or progress – to mean “additional” or “to a greater extent.”
However, the dictionary does say that further can also be used for physical distance:
Finally, the word furthermore means “in addition” and is used to introduce a phrase:
The word furthermore is a bit formal, and when speaking everyday English we would usually say “Plus” or “Also” instead.