I've been using this hack for a while now. The main issue with it is that it completely messes up on pages that resize themselves. It looks really ugly when that happens.
I never understood the point of this kind of minimalism. I have 1200 pixels of vertical space. I prefer to have a real set of menus and control buttons up there in addition to a full width tab bar.
Keep in mind that the option to `Always show the tab bar` needs to be checked. Otherwise, the orange Firefox menu button will overlay the navigation bar if you are only viewing one page, and you won't be able to click on it until another tab is opened.
Also, here is a link to CSS code that, in addition to pushing the tab bar up, makes the orange menu button look much nicer:
If you are after real estate isn't the best option the full screen mode (F11) plus auto hiding tabs, that way you can have the whole screen for the web page and still use tabs...