Good addon; it does what it promises. What would be a great addition is a hotkey to pop-up/merge back the current tab (even better if customizable, but a fixed hotkey would be ok too). This should be helpful for power users who like a minimalist UI and don't want to touch the mouse at all if possible.

It is not perfect however; sometimes the popup window which is created cannot be maximized, and the merging back into the main window behaves weirdly when you have multiple normal browser windows open. This is probably not the developer's fault and somehow related to WebExtensions, but it would be nice if those kinks where ironed out in the future.