Limitation: the add-on does not modify the data about the OS that is exposed by Flash or Java (if you have one or both of them installed and enabled); while most sites probably don't use Flash or Java specifically for determining a user's OS, it is quite possible that some do.
Since 18.0t17.0.2 release, Windows (such as Vista or XP) installations are blended in by default as well (as 7).
If you'd like to unblend in for a while (or re-blend in later on), please filter for the preference containing notApplicableIfRealPlatformIsWindows (on Windows) or applicableIfRealPlatformIsNotWindows (on non-Windows) in about:config, and toggle that preference's value.
The add-on has no menu items or buttons at the moment: its value and functionality are provided seamlessly by the component behind the scenes. Modifications made by the addon are logged in the Error Console (as well as dumped to the regular console).
If the add-on is disabled or uninstalled, all modified settings are restored by the add-on to their original values.