Depending on the HTML tags and style rules, there are some blocks of content that Firefox can display normally on continuous media like the screen but which it is unable to paginate, so the part that doesn't fit on the page where it starts is just not printed. Sometimes there is a way to modify (hack) the page so it prints correctly. Sometimes there are other workarounds, like converting to PDF or capturing a screenshot and printing that. Sometimes... you have to use another browser. Is there a site you need to print from regularly that has this problem?