Question on CS5

I'm on Mac using photoshop CS5. Perhaps I hv accidentally pressed a button and I do not know how to revert back to the old style
Previously when I press F key, the screen will toggle from full screen to smaller screen and so forth.
Now when I open CS5, the F key doesn't toggle like as before. Instead I can see other working files in the back grd. Its kind of merging my photo that has been opened in CS5 and I still can see other files in the back ground.

Any advise how to revert back. Tks

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David