Forum Moderators: open
I'm not certain I understand your question completely: are you saying that you have built a site which is supposed to be 100% in width, but in Firefox the width is greater than the width of the window, thus forcing a horizontal scrollbar?
If so, it can be difficult to diagnose the cause without seeing a sample of your markup. Are you using a 100% width table or another method? Could you post a short, relevant snippet of your markup with any specifics removed? (See the guidelines for posting code [webmasterworld.com] for how to prepare your markup.)
You should also check your markup with the HTML validator [validator.w3.org] which may help identify the cause of the problem.