This is how it worked in 0.8...don't know if the Preferences panel has changed in 0.9 or not:
Tools -> Options..., and choose "Advanced" in the left column. Make sure that "Open links in the background" (under "Browsing") is checked. When you middle-click or Ctrl+left-click on a link, it will open in another tab, but keep your current tab in the foreground.