It works without problems on WinXP, there is no difference; throughout the beta-versions Firefox incrementally used some minor tweaks of Chrome (for instance, where the link-bar on mouse-hover or the status-information on loading the web-page was displayed). Sad to hear that it does not work on all OSs -- for me it is one of the most important improvements.
EDIT: oh, found it, but my tabs were already on top.
Problem is, Chrome also does away with the title bar.