How is this not a default feature?!!? Seriously: I hunted for this or a similar hover/preview add-on. This is perfect! Exactly what I envisioned! The hovered previews have scrollbars, they are nested, you can easily move back to the nested level you wish to.... Perfect. :)

Add a way to make it enable for every website without separately clicking it and a blacklist to disable it from some sites

Great idea! I liked the nested previews approach.

Unfortunately this is also limited by CORS policies, the same as other similar iframe-based addons, which leads to not all pages being available for preview – but I guess this could not be resolved until such a feature will be implemented in the browser itself.

My suggestion would be to show different 'Open in background tab' and 'Open in foreground tab' actions. Also would be great to see this extension open-source, and option to reveal hover tooltip only when holding the modifier key (CTRL or Shift).