Tools,Add-ons,Extensions, do you have "Fox Splitter" or "Split window" extension? If so, does disabling it and restarting Firefox work?

Make sure that you do not open a bookmark in the sidebar.

Opening in the side bar is the default for bookmarks that are created via a link or button on a website.
You can check the Properties of a bookmark via the right-click context menu in the side bar (Ctrl+B; Cmd+B on Mac).
In the Bookmarks Manager (Bookmarks > Organize Bookmarks) you can click the More button in the Details pane at the bottom right.
Make sure that "Load this bookmark in the side bar" is not selected.