

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.
Yup dumdum, I’ve tried to explain that Zen also has this expanding ‘thing’, but unlike with Firefox it can be placed opposite side of the vertical tabs. If you for instance press Ctrl+B and you get the expanding thing (sidebar) with bookmarks. Just like in Firefox it expands and reduces webpage space. The overlay is something else, called the web panel. Something Zen introduced which is additional to the sidebar and not a replacement of it. I’ve even taken the time to show it in a screenshot, but apparently that makes me a smartass.