The feature actually was there since Firefox 4, but wasn't default. In Firefox 4 it was an about:config setting, sometime later a checkbox was added to Firefox

The feature actually was there since Firefox 4, but wasn't default. In Firefox 4 it was an about:config setting, sometime later a checkbox was added to Firefox preferences (General, Don't load tabs until selected). I've been using this feature for as long as it has existed, and with the way I use tabs it has been a great help to me.