A couple things Rated 4 out of 5 stars
Interesting idea. I'm honestly not sure that I'll keep it for long, but I'm sure others will. As others have said, it stops the Firefox button (orange) from working when maximized. If I "restore" the window it does work.
Also, I would prefer it show the location bar when I mouse over the tab bar "area", not just active tabs. If I have one or two tabs open, I have to aim for them to get the location bar back. If the whole bar triggers it, then I just toss my mouse to the top of the screen and it works.
The Firefox button should be working since v2. Also v3 makes it so pointing at the empty tab area will correctly work on Windows.