

It’s configurable from the sidebar dropdown menu, e.g. when opening your bookmarks (Ctrl+B):
But you can also use this about:config
setting:
Again you keep calling it a popup, but it really isn’t. It does not pop up or overlay the browser viewport, it sits on the right and pushes the viewport left reducing its width.
A popup is overlay / in front of your viewport or UI, the sidebar is not in front of it. If what you see is (like a) popup, you’re talking about the web panel, which a different concept Zen added. Indeed in Firefox the vertical tabs are part of the sidebar and thus they can’t be move independently. In Zen, the vertical tabs are NOT part of the sidebar, and thus you can move the sidebar to the right while the tabs remain on the left.