<a href="http://www.accesstoinsight.org/tipitaka/an/an05/an05.177.than.html">Right livelihood</a> in the old scripture is merely guidance from 2,600 years ago. Today, the world is far more complex. 

In summary, the Buddhist standard for any behaviour is avoiding harm. For example, when <a href="http://www.businessinsider.com/how-george-soros-broke-the-bank-of-thailand-2016-9?IR=T">George Soros</a> shorted the Asian currencies back in <a href="https://en.wikipedia.org/wiki/1997_Asian_financial_crisis">1997</a>, if this harmed those economies (causing people to lose jobs & money) then this trading in currencies was certainly unethical from a Buddhist point of view. 

For the average small person, carrying on a genuine educated skilled business of trading in foreign currencies (forex) is not unethical because it is not harming anyone. 

While such trading is not an "investment" in productive assets (such as investing in the stock market) and thus the other side of a winning forex transaction loses, both parties are playing the same game & taking the same risks therefore it is not unethical. 

However, this said, it is not the type of business mentioned in the Buddhist scriptures because, as I already mentioned, it is not an "investment". It is only a business that benefits oneself (despite it not harming others).