Mrs. Fitzgibbons (whom her husband, Farmer Fitzgibbons, refers to as "Beth" in one scene in the movie), is a hardworking house wife, seen managing the household chores while Farmer Fitzgibbons attends to the matters of farm work. A dedicated, honest and generally stalwart woman, but still more than a little unnerved by the prospect of rats infesting her beloved rose bush, and assuming, after receiving a telephone call from NIMH, that the rats might have some terrible disease.
Quote: "Dragon! You lazy kitty. Go catch some of those rats out in the barn. I've never seen a cat sleep so much."
Origins of the character: In Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", we don't get much of a description of Mrs. Fitzgibbon (the family name has no "s" on the end of it in the book), but she pretty much comes across as more or less the same as the character in the movie. However, there is more attention devoted to her concerns over losing the rose bush when NIMH comes to investigate the nest, and has Farmer Fitzgibbon actually bulldoze the bush, utterly destroying it (while Billy stands on the porch and cheers).
In the animated sequel: Though we do see the Fitzgibbons farm in the sequel, none of the Fitzgibbons family itself are ever seen during it.