Are you sure? I do remember a mockup from that timeframe that showed all platforms on one page, and was static rather than interactive. Given the greater sophistication in these and the fact that they're still linked from their wiki (https://wiki.mozilla.org/Firefox/Features/Theme_Refinement_a...) I had assumed these to be recent.
Hmm, in the new ones I was derailed by the icons of background tabs being not lined up with the OSX color pills. Yet the old mockups show a curve similar to the tab shape that creates a very welcome break in the design, explaining that "here are the tabs", and "here are the window controls", while the curve slant explained the height position difference between the pills and the icons/text (notice how the line linking the green pill center to the home icon center looks perpendicular to the centermost part of the separator curve, which cuts this segment in half). Without that curve, the new design simply feels broken. What's worse, since it's right there on the top-left, this is the absolute first experience of the software from the user's eye. It does not feel robust at all.
Edit: The older set I was referring to: http://people.mozilla.com/~shorlander/ux-presentation/ux-pre...
Edit2: The OP links were added to that wiki page in an early March 2012 revision.