there is some misunderstanding here. there is no fullscreen safari in iOS5. address bar disappearing is not full screen.
in ios6 to make the safari goes fullscreen, u have to press the little icon arrow in the bottom right.
when it goes fullscreen, there is no tabs button. so to switch tabs u have to turn your phone into portrait, then press tabs button. or u just turn fullscreen off and press tabs button, after that turn full screen on again.
i prefer to switch to portrait then press tabs button then switch to landscape again. it's a bit faster.