There's an extension called "Menu Editor" that allows you to edit the context menus in Firefox. It works, but it can be confusing to use, because the context menus are... so contextual.

In your case though, you could just swap the places of the "New Window" and "New Tab" menu items.

Edit: here's a link to the extension https://addons.mozilla.org/en-us/firefox/addon/menu-editor/