Place the broth, fruit juice, or water; celery; onion; and butter or margarine in a 2-quart microwave-safe casserole dish. Cover and heat on high (100%) power for 5 minutes, or until boiling. (Note that cooking times may have to be increased or decreased, depending on the power of your microwave.)
Remove the casserole from the microwave oven and stir in the stuffing mix. Cover and stand for 5 minutes.
Return the casserole to the oven and heat on high (100%) power for 1 minute, or until the liquid has been absorbed and the stuffing is hot. Fluff with a fork before serving.