Mrs. Frisby is practical. Instead of bemoaning her hardships and feeling sorry for herself, she immediately takes action to solve her problems. 

Mrs. Frisby is loyal.  When Billy Fitzgibbon captures her and she learns that the other animals, including the rats, are in danger, she warns them.  She could have just selfishly thought about her own family, but instead she makes the other animals' safety a priority, because they have helped her.

Mrs. Frisby is also open-minded.  She knows that she simply cannot afford not to consider getting help from other sources, like the rats and eventually the old owl.  She looks past the exterior surface of the other characters.