The blue icon to the top left shows that this is Minefield[1] (codename for nightly builds of Firefox). Also, it has a dropdown arrow, showing that this is probably using the Hide Menu Bar[2] addon. The lack of usual menubar/chrome of the browser can be explained by the use of Awesomewm[3] or dwm[4]. Firefox 4+ builds all have the same tabbing as in the screenshot. Also, the bookmarks toolbar has been dragged to the same line as the addressbar and all bookmarks and bookmark folders have no name, thus only showing their favicons, which is the default behavior. Also, the grandparent comment has apparently moved the status bar to the top, on the same level of the tabs. This is new to me, and maybe the default behavior in current nightly builds of Firefox.
If I may be so bold, I'd wager this author is using Arch Linux, since awesomewm is very popular with that group of users, and because of the rolling-release cycle nature of Arch, resulting in the latest builds of Firefox.
[1]: http://www.mozilla.org/projects/minefield/
[2]: https://addons.mozilla.org/en-us/firefox/addon/hide-menubar-...
[3]: http://awesome.naquadah.org/
[4]: http://dwm.suckless.org/