It works, but... Rated 4 out of 5 stars
when dragging from the top to un-maximize, the window doesn't go back to its original size. It instead fills the screen height and becomes square. GNOME 3.10.1, Firefox 25.
Edit: Updated to 2.5, but still have the problem. Legacy mode checkbox doesn't seem to work properly. Bug reported: https://github.com/seleznev/firefox-extension-htitle/issues/22
Is "Use legacy mode" checkbox enabled (on preferences page)? It should be unneeded if you use GNOME Shell.
Also, version 2.5 works better (https://addons.mozilla.org/en-US/firefox/addon/htitle/versions/?page=1#version-2.5).