Hi Ruby.
The definition of firm-fixed-price contracts is found on page 471 of the 6th edition of PMBOK and states as follows. "Firm fixed price (FFP). The most commonly used contract type is the FFP. It is favored by most buying organizations because the price for goods is set at the outset and not subject to change unless the scope of work changes."
From the plain reading of the above text, we can conclude that the costs in executing an FFP contract remain the responsibility of the contractor as long as the scope remains the same.
I hope that helps. I wish you all the best on your exam.