Please download latest version from: https://addons.mozilla.org/firefox/addon/NewGlasser/versions/
Please note that the addon will work only on systems with the Aero Glass graphics style (Windows Vista, Windows 7). On older systems it won't work (Windows 2000, XP, Server 2003)!!!
In Windows Vista, while you maximize the window, transparency of the window disappears. To prevent this, install the program VistaGlazz (http://tinyurl.com/vistaglazz-download). After installing, click the second icon and select Patch Style.
If you want to have transparency in download manager also, download this addon - https://addons.mozilla.org/pl/firefox/addon/glass-window-download/