

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.
The Tesseract web app does that. It stacks posts in the feed that have the same url or title.