An efficient capital market is a market that reflects all available news and information. An efficient market is also quick to absorb new information and adjust stock prices relative to that information. This is known as an informationally efficient market. Generally, efficient markets are expected to reflect all available information. If that is not the case, investors with the information may benefit leading to abnormal returns.
There aren't many (or any ?) methods for a retail investor to do forex trading over the weekend. There was one, OANDA, but they claimed the low volume was too challenging for them where they would be taking on too much risk having to take the other side of the trades:
Of course, there is a lot of forex trading over the weekend -- just not by you or I. Central banks, corporate and institutional (government) bankers, multinational corporations and more are all trading 24x7.
So the market is efficient for them. The rest of us stuck with a losing forex position thanks to news that occurs over the weekend are stuck with that increasingly deteriorating position until the markets open on Monday (or Tuesday, when Monday is a banking holiday like what happened this 3-day weekend.)
That's not the definition of an efficient market. Market-changing information occurs over the weekend as well (with announcements oftentimes timed specifically for release over the weekend.) Bitcoin markets don't have this restriction. That still doesn't mean bitcoin exchanges can be considered "efficient" though. For instance, there was a pattern called the "weekend dip" in which buyers interested in buying had insufficient cash at the ready at the exchanges in order to buy and as a result there was little buying to counter a weekend selloff until new cash arrived at the exchanges when funds sent through the banking system were credited. Lately, however, the weekend dip opportunity seems to have abated.
At some point a financial company will start using Bitcoin as the method for moving value in and out of other assets, including offering the ability for that to continue throughout the weekend as the way to differentiate that investment offering from the competition.