Right livelihood in the old scripture is merely guidance from 2,600 years ago. Today, the world is far more complex. Many businesses not mentioned by the Buddha are obviously wrong livelihood.
In summary, the Buddhist standard for any behaviour is non-harming. For example, when George Soros shorted the Asian currencies in 1997, if this harmed those economies (causing financial & job losses) 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; even when it is only a business that benefits oneself.
As for how large scale FX trading can harm an economy, read here: Forex Trading: Impact on the Dollar and the Economy
Even without outright price fixing, traders can create asset bubbles in foreign exchange rates. It may have happened with the U.S. dollar in 2014 and also in the last quarter of 2008. A strong dollar makes U.S. exports less competitive. It slows GDP growth. If traders bid the dollar down, then oil-producing countries will raise the price of oil as oil is sold in dollars. The impact of expanding forex trading needs to be better regulated in order to avoid potential bubbles and busts.