can not install gnome shell extensions

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http://askubuntu.com – I upgrade from 10.04 to 12.04, install gnome 3 and remove unity. My gnome version is 3.4.1. The GNOME Shell extensions is installed and enabled. When I use firefox to visit each extension page in https://extensions.gnome.org/, for example, https://extensions.gnome.org/extension/5/alternative-status-menu/, there is no where to install the extensions. I am told that one can install extensions by visite these extension pages. (HowTos)