You can't currently autohide the panel in Unity. It is also not a planned feature.
There are ways you could possibly make the panel autohide:
Post an idea on Ubuntu Brainstorm to add this option and hope a developer sees it and implements it or creates a for with this feature.
Report a bug on Launchpad stating that this option should be available and hope a developer sees it and implements it or creates a for with this feature. This will probably get marked as opinion or invalid or at best a wishlist bug.
Get the code and add this option then merge with trunk or create your own fork.
Best Answer
Using the GNOME extension called Hide Top Bar.
Configure the extension in your Firefox or with Tweak Tool