Is it okay to use the front brake while going downhill?

Yes. In fact, you’ll need it if you want to really control your speed.

Misusing or over-applying the front brake can send a rider over the handlebars. That’s why some cyclists believe they should rely only on the back brake. But when much of the weight is on the front wheel, it’s the front brake that will provide the most stopping power. To effectively control your speed on a downhill, you need to shift your weight back (even to the point of actually placing your butt behind the seat, on a really steep descent), then use both brakes.

The rear wheel may skid in such situations. Once it’s skidding, it’s not providing much stopping power. As soon as you detect a skid, briefly release the rear brake and gently re-apply it.