I just tried the userChrome.css hack and it breaks a lot of stuff. Tree Style Tab stops being able to tell which tab is active, my address bar only affects the first tab, and my window buttons are all blank. Looks like I'll have to wait for the tab hiding API to try this out.

edit: Nevermind. I was just using the wrong hack. Using this one: https://news.ycombinator.com/item?id=15870894 + using Customize to reenable the title bar works great.