Are you trying to install it from here? https://addons.mozilla.org/en-US/firefox/addon/4441/ Your UserAgent string in Firefox is messed up by another program that you installed, and the addons website probably doesn't know you are running Firefox 3.6.2 . [http://en.wikipedia.org/wiki/User_Agent] type '''about:config''' in the URL bar and hit Enter ''If you see the warning, you can confirm that you want to access that page.'' Filter = '''general.useragent.''' Right-click the preferences that are '''bold''', one line at a time, and select '''''Reset''''', Then restart Firefox