the newtab page changes the undo behavior of newly created tabs.
1. open any page
2. create a new tab
3. press undo
the url of the initially loaded page should immediately fill the urlbar.
the urlbar is populated with about:newtab
pressing undo a second time correctly populates the urlbar.
Interesting that you actually use this, never was an intentional feature :)
Fixed by bug 725200.
Mozilla/5.0 (X11; Linux x86_64; rv:13.0a1) Gecko/13.0a1 Firefox/13.0a1
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:13.0) Gecko/13.0 Firefox/13.0a1
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/13.0 Firefox/13.0a1
Verified on latest Firefox Nightly with steps in comment 0.