An error? I'm guessing Omniture is referring to Mozilla Firefox, which is I think the #1 browser used today and is not on your list. Firefox 4 is the current release.
Additionally, Firefox 4.08 is the current Beta version available for those who enjoy testing possibly unreliable software 🙂
Interestingly, I don't believe Gecko is classed as a browser, rather as a function within a browser:
Gecko's function is to read web content, such as HTML, CSS, XUL, and JavaScript, and render it on user's screen or print it. In XUL-based applications Gecko is used to render the application's user interface as well. Gecko is used in many applications, including a few browsers, such as Firefox, Mozilla Suite, Camino, etc.
Very odd.