How can I force new windows to open in background & without focus?
Solution 1:
I think this is a bug, so I reported it. This used to work with Gnome 2, it works with Gnome Shell and Gnome 3 fallback. I suspect compiz, but what do I know.
Solution 2:
Right-click on the window title bar of Liferea and choose "Always on Top". Now all your links will open in the background.