You are running a no longer supported Firefox Firefox 4.0b12 version.<br /> Please update Firefox to the latest version via "Help > Check for Updates or Help > About Firefox" You can find the latest Firefox release here: *Firefox 9.0.x: http://www.mozilla.com/en-US/firefox/all.html *http://kb.mozillazine.org/Installing_Firefox#Linux ---- See also: *http://kb.mozillazine.org/Unable_to_install_themes_or_extensions *https://support.mozilla.com/kb/Unable+to+install+add-ons