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.
But also that's been part of Wikipedia's website for years now, no special browser support necessary. And because it's tailored specifically to Wikipedia, it works great!
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.
I used to appreciate it (on Wikipedia), but too often it gets triggered by accident and prevents me from reading what I actually want to read. So much so that I had to find some way to disable it (probably by blocking JS via uBO).
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).
