That's strange as it still works fine for me. That about:config setting puts things back to how they were until a few weeks ago, ie it gets rid of the new big previews but leaves the more discrete tool-tip that's been there since Noah was a lad.
I absolutely agree that there should be a simple On/Off in the user settings and that "Make Firefox Your Own" and such slogans become less tenable with each up-date.
