Firefox 3 unified button not unified

Curiosity

Registered
With the theme Pitchdark 3.0.2 used in FF3 on a Mac, the unified back-forward button is split into 3 pieces, with extra dropmarkers inserted between the pieces. I was able to get rid of the markers themselves by adding extra code to browser.css, but the gaps remain. How do I re-unify the button?

Why does this not happen with Windows Firefox 3?
 
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