This is neat, but I feel like Opera had this waaay back like 15-20 years ago. My memory could be wrong though, I stopped using in favor of Firefox a looog time ago
-- edit --
At least since 2015 https://blogs.opera.com/news/2015/07/cool-functions-tabs-in-opera-computers/
-- double edit --
Since 2008! And according to that link, apparently Firefox 3.1 had tab previews back then as well?
https://www.rarst.net/software/enabling-tab-preview-in-opera/