Sorry, this happens on many sites. Firefox handles certain style rules just fine for screen, but when asked to split the content into separate blocks for printing, it treats certain blocks as "unbreakable": it gives them as much space as it can, such as its own page, but the excess just runs off the bottom. Is there a URL you can share for more specific diagnosis? There is work in progress this year to clean up these kinds of problems, but for the moment, you may need to "hack" the page to make it print correctly. I created an add-on to address this problem, it has various buttons you can try to fix page breaks. Not perfect, but might be helpful nonetheless. If you want to take a further look: https://addons.mozilla.org/firefox/addon/printable-the-print-doctor/