When exiting customization mode, the browser first resets the padding on the window and then jumps back to the previous tab. Between the end of the animation and the jump back, a blank page is briefly shown causing a white flash (before the customization tab finally closes). This happens on all platforms, but it is much worse on Mac than on the others. Bug 964257 might be related here, as it happens in the exact same time frame.
I think this will be fixed by the patch for bug 964257…
See Also: → bug 964257
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