Had to discover that command click opens the bookmark in browser. For a _bookmarking_ application I was very confused for a moment before I figured that out.
It seemed ridiculous to have to click on a tiny little open icon for what I have to imagine is the primary user flow.
The preview and edit pane are great. To avoid any new options or having secondary actions, maybe try both opening the browser _and_ the new preview tabs at the same time. If that doesn't feel quite right, then I would make a big clickable area for opening preview on off on the right side or making _that_ *alt* + click or something.
Gmail + preview or just selection is a good example of important secondary functionality not getting in the way of primary action and not having to add a ton of new shortcuts or options.
Edit: The above is still valid, but I didn't actually get what was going on when I clicked on a bookmark. Didn't realize it was an embedded browser. Was looking at a very non-interactive site that looked like a preview image. Moving the URL to the top would go a long way in making it feel like a browser and be more obvious. Doesn't change the suggestion above - maybe the weight of it though.