What HUD can do :
Instead of a nested tree of options, users will have a "vocabulary UI" that's "closer to the way users think" according to Shuttleworth. .
So instead of having the traditional "File," "Edit," "View" menus (and so on) we can pull down a menu to type in commands. So if I want to change preferences I'd start typing "pref" and (in theory) the HUD will suggest commands that matches that.
Which probably says it is an Integrated Menu Utility of active Application . Since Firefox menu don't have any such MENU option like
Go to URL or
Enter the URL you want to Navigate, so currently in present scenario we can't go to desired URL from HUD.
According to Unity/HUD Wiki Page
HUD is in a very early stage of development, and not ready for
So if you are interested in developing such features , you probably can by joining the Launchpad and getting involved in that project.
For further reading and references here :Link1, Link2, and Link3.