Firefox doesn't have a "split-screen". Unless you installed an add-on to add that feature to Firefox. Perchance is one of your Bookmarks opening in the Sidebar? If so, open '''Properties''' for that Bookmark ''{right-click and select Properties}'' and de-select '''''"Load this bookmark in the sidebar"'''''. That 'box' can become check-marked when using a "Bookmark this Page" 'button' on a web page; the JavaScript for that type of 'button' isn't fully compatible with Firefox. So don't use a 'button' on a web page to save a page as a bookmark, use the "Bookmark this Page" feature that is inside of Firefox instead.