Furnished, unfurnished and even semi-furnished flast are available in Edinburgh, it just depends what your budget is and how much junk you've brought with. Keep in mind there's an IKEA just outside the city for all your home furnishing needs. Also, be mindful that furnished can mean anything in terms of inclusions and style.
A letting agency can be superhelfpful if you're a first-time renter; and they can also shed some light on the different areas around town. For example, rental prices in Leith can differ dramatically when compared to New Town. You'll get the best deals if you sign a long-term let - which last either six months or one year.
Enjoy the festivallllll!!!!