In the Vanijja Sutta (AN5.177), there is defined what kind of businesses a lay person must not indulge in:
"Monks, a lay follower should not engage in five types of business.
Which five? Business in weapons, business in human beings, business in
meat, business in intoxicants, and business in poison.
This does not include currency trading or other kinds of speculative trading.
In both the Dighajanu Sutta and Sigalovada Sutta, gambling is criticized by the Buddha and he states many negative consequences of it.
"These are the six dangers inherent in compulsive gambling: winning
breeds resentment; the loser mourns lost property; savings are lost;
one's word carries no weight in a public forum; friends and colleagues
display their contempt; and one is not sought after for marriage,
since a gambler cannot adequately support a family.
On the other hand, the Dighajanu Sutta also seems to be fine with trading:
"Four conditions, Vyagghapajja, conduce to a householder's weal and
happiness in this very life. Which four?
"The accomplishment of persistent effort (utthana-sampada), the
accomplishment of watchfulness (arakkha-sampada), good friendship
(kalyanamittata) and balanced livelihood (sama-jivikata).
"What is the accomplishment of persistent effort?
"Herein, Vyagghapajja, by whatsoever activity a householder earns his
living, whether by farming, by trading, by rearing cattle, by archery,
by service under the king, or by any other kind of craft — at that he
becomes skillful and is not lazy. He is endowed with the power of
discernment as to the proper ways and means; he is able to carry out
and allocate (duties). This is called the accomplishment of persistent
The Sigalovada Sutta is also fine with investment in business:
By dividing wealth into four parts, True friendships are bound; One
part should be enjoyed; Two parts invested in business; And the fourth
set aside Against future misfortunes."
So, the question is, would currency trading and other types of speculative trading be considered a form of gambling or a form of trading or a form of investment?
If it is gambling, then it is not good. If it is trading (like trading of goods e.g. retail sales) or a kind of investment, then it is alright.
So, which is it? This is open to interpretation, and is probably best analyzed by the experts at the Economics Stack Exchange.
Regarding the point that currency trading may be harmful to the economy - this is similar to the topic of meat-eating. Business in meat is wrong livelihood, but eating meat, that was bought dead and frozen from the supermarket, is not wrong for lay people according to the Theravada tradition. Please see this question for more details.
Similarly, if an individual does not have any direct intention to harm others, but the action indirectly affects others, this is not wrong but may not be preferable.
Also related, is the story of the hunter's wife from Dhammapada 124. Please read this answer for more details.