Joanna K. Horsnail has liked Mayer, Brown LLP since joining as an associate in 1998. She didn’t know the degree to which the firm liked her until 2003. Horsnail — a construction and government transactions attorney — was on maternity leave for her first child and told the firm that when she returned, she wanted to come back at 70 percent of her previous time and salary commitment. The firm responded by promoting her to partner. “That was one of the first major supports they showed to me,&rdquo …