Some of my rabbit’s droppings are very round, some are stuck together, they’re black, some are a bit long, and some look like they’re connected by fur

Normal rabbit droppings are usually cylindrical or slightly flattened ovals, fairly firm, uniform in size, smooth on the surface, and dark brown or black in color. Because rabbits are herbivores, the fur-like bits in the middle are most likely bits of grass stem, and since rabbits frequently groom themselves, some hair ends up in the stool — all of which is normal.

But from the photo, your rabbit’s pellets are rather small, which may indicate slightly insufficient water intake. Make sure it always has plenty of water available, and occasionally offer vegetables such as carrot or celery to supplement moisture.