It was possible to open recommended pages in a new tab. This is one of the things we tried at StumbleUpon's quasi-successor, Mix. It was significantly less engaging for new users than having everything in one tab. Some people who had used the real StumbleUpon didn't just find it less engaging; they hated it and sent hate mail to make sure we knew. It was a distinctly inferior experience.

You can no longer deliver the full StumbleUpon experience through a web browser, at least not legally. The tightening technical constraints make it impossible to replicate what we could do 10 years ago. Of course now it's also harder for someone to enroll your machine in a botnet because you played a game, or steal your passwords because you installed a browser extension, so those things are to the better. But increasing attention to security and control also make it harder to offer "fun and weird" experiences of various kinds.