I'd be a little more optimistic than some. I grow a lot of Douglas Fir on my tree farm. Firs have a relatively fine root system compared to pines, and do well if the soil they are in is well drained. It sounds like you did not do anything to disturb the root/soil condition in a way that would change the support for the tree. The greatest risk for that tree, being that it is so large, is wind. I'd say if the prevailing winds are from the cliff side, and the tree is in the open by itself, it may come down sometime in a wind storm. ie root support on the cliff side are not as strong and it then will be falling away from your pond. However chanches are the prevailing winds are tward your pond or the tree would not have survived as long as it has. Time will tell. The Doug Fir is a great tree; Enjoy it. If it is shading your new pond it will help keep the water cool for the fish, if you should want to stock it. Keep the grass and other vegistation on the cliff to prevent soil erosion.
