It's an idea that makes a lot of sense on Wikipedia, because every page there explicitly starts with a short summary, and often an image that can be pulled from the infobox. I'm not yet convinced it will work well on the rest of the Web.
Page preview seems nice in theory, but I'm unconvinced it'll be that useful. Web pages just don't have a the level of standardization necessary to automatically grab a useful preview. And I don't think Firefox has a big enough pull to make that sort of standard.
Similar story with GitHub "hovercards". They were helpful, but someone recently decided they have to work in the Issues/PR lists too and, again, it just prevents me from reading the very thing I want to read. Had to disable them (everywhere, unfortunately).