When using Firefox on Windows 7, some sites, may not work, because of he maximum windows version, which is usually set to 6. Windows 6 is Vista, Windows 7 is 6.1 with a build number after it. Well today I tries to stream the new Green Day album from http://www.rhapsody.com and I got just such an error. Unknown OS, cannot play. You can easily fix such errors with an addon, Configuration Mania. (https://addons.mozilla.org/en-US/firefox/addon/4420) Simply go into the preferences for Configuration Mania, and go to the User Agent settings, and pick a previous operating system. for my issues, I picked windows XP, and instantly rhapsody worked. You can also pick IE or safari for some issues you may have with sites. This does not change the rendering engine, but rather changes the information sent to websites, about your computer and browser.