- ⌘ + L
- ⌘ + Enter
How it works: The first step "Highlights the URL." (File > Open Location...). The second step "Opens the URL in a new background tab."
I'm usually a Chrome user, but I just tested it. It works on Safari, too. I can't find a reference for Safari, but here is that for Chrome. (under Mac > Address bar shortcuts)
Edit: TrautsTrauts beat me to it while I was formatting the key glyph. Feel free to ignore my answer and upvote him!