Line a 9×5 inch baking pan with plastic wrap and set aside. The plastic wrap makes it much easier to unmold, as you don’t have to fuss with submerging the finished terrine into hot water.
Cut up the citrus fruits and set aside.
Allow gelatin to soften in 1/3 cup water. Use a medium sized bowl, enough to hold about 3 cups of liquid.
Boil the other 1/3 cup water with 1/3 cup sugar; boil for 5 minutes and pour it into the gelatin mixture to dissolve the gelatin.
Next, pour the orange juice into the sugar-gelatin mixture. Leave this in the fridge for about 2 hours. You want the solution to be thickened but not set.
Drop the fruit into the thickened gelatin mixture. Stir gently and then pour everything into the previously prepared baking pan lined with plastic wrap.
Leave it in the fridge to set overnight.
Unmold onto a rectangular plate, wiggling the plastic wrap a bit if you have to.
Using plastic wrap sometimes leaves wrinkly marks on the outside of the terrine. If you'd prefer it without those marks, you can do one of two things: trim off the sides with a hot knife or use an unlined pyrex dish, but you would need to unmold by dipping the dish into hot water.