At the beginning of Act Two, John and Elizabeth Proctor briefly discuss their servant, Mary Warren, who has been participating in the witch trials. When Elizabeth tells him that Mary Warren has gone to Salem that day, he becomes angry because Elizabeth had heard him forbid Mary Warren to go. When Elizabeth claims that Mary Warren "frightened all [her] strength away," he asks, "How may that mouse frighten you, Elizabeth?" John Proctor uses the metaphor of a mouse to describe Mary Warren's typical behavior. We think of mice as skittish creatures, quiet and small and frightened, and this is how Mary Warren has always acted.
Now, however, she "is a mouse no more," according to Elizabeth. Mary Warren has developed quite a sense of self-importance now that she is "an official of the court," and she feels her obligation to the court outstrips her obligation to her employer. Prior to her change in status, we can assume that she would have willingly deferred to either John or Elizabeth, but now that all the accusing girls have been placed on a pedestal in town, Mary Warren is no longer quiet and small and frightened. She has been given a voice and made bold by the way the judges and ministers now all defer to her and her friends.