Story at New York Times. There is speculation that Mayer Brown was the firm. Excerpt:
A top-secret document, obtained by the former N.S.A. contractor Edward J. Snowden, shows that an American law firm was monitored while representing a foreign government in trade disputes with the United States. The disclosure offers a rare glimpse of a specific instance of Americans ensnared by the eavesdroppers, and is of particular interest because lawyers in the United States with clients overseas have expressed growing concern that their confidential communications could be compromised by such surveillance.