Safari does it, Chrome does it, and Firefox did it up to version 12: put the focus automatically to the location bar (a.k.a. address bar) when opening a new tab.
With FF13 this behaviour changed, probably due to the new New Tab Page. It doesn't matter if the newtabpage configuration is enabled or not, when you press Cmd-T (or Ctrl-T) the focus is nowhere and you have to press Cmd-L before you can start typing.
Is there a configuration option to revert to the old behaviour?