Elizabeth is referring to her servant girl, Mary Warren with this statement. Elizabeth's husband John Proctor had forbid Mary to go to Salem because of the trials taking place. While John was out, Mary informed Elizabeth that she was going to the trials because she is now an official of the court. Previously, Mary Warren was a meek, shy girl who was often overlooked and no one really paid much attention to her. John Proctor refers to Mary Warren as a mouse. Once Mary gets a taste of attention through her involvement with the witch trials, she stands up to Elizabeth, leading her to comment that Mary is "a mouse no more". Mice are meek, timid creatures, and the metaphor fit Mary's personality before she became an "important" member of the court.