Err it's pretty obvious, when you click on the menu there is a giant button that says Windows 8 Mode on it.
1) I didn't even bother to install Firefox on my Windows 8 tablet because I didn't know about this feature; had I known, I would have.
2) http://support.mozilla.org/en-US/kb/how-do-i-launch-firefox-... seems a far cry from a "giant button" - not only do I rarely if ever use that menu (because keyboard shortcuts) but I doubt I'd have picked up on that option unless I were looking for it.