Not currently working with Craigslist, my main need. I get "There was an error loading this page" in each hover-box there.

Where it does work, e.g. at Rhymezone, need is to have a bigger hover-box. But making it bigger only lasts for that hover. Opening another hover-box from the very same page reverts to using the tiny box! How to make the bigger box setting stick for a site?

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).