my 18 year old daughter has a firm lump, the
I understand it is concerning to notice a lump at this age. Therefore I'd suggest you get it evaluated at the earliest.
A simple ultrasound scan of the lump will provide us with enough clues to know about the nature of lump and whether or not it requires attention. Your local care doctor will be able to order this test. Alternatively, please visit a general surgeon who will be able to help in diagnosing and managing it.
Hope this answered your question. Let me know if you need clarifications.