Being firm is critical in achieving what you need out of negotiations. However, if you enter negotiations without room to concede or be flexible, your negotiations will fail.
You must separate those items that you desire into absolute needs, strong desires, and nice-to-haves. The company might meet every requirement but one, and your ability to be flexible and creative will be the key to negotiation success.
You cannot expect a company to bend at your every request if you are unwilling to budge at any of theirs. Also, don't overstate your qualifications and salary requirements assuming that a company will take a bit off whatever you state. Most employers readily recognize this as amateurish. State what you want as what you want.