Firefox 5.0 is the most common nowadays; XP is also still the leader; So, the in...

Firefox 5.0 is the most common nowadays; XP is also still the leader; So, the intersection between the two is probably high. According to UserAgentString, there's only one string (well, two variations):

    Mozilla/5.0 (Windows NT 5.1; U; rv:5.0) Gecko/20100101 Firefox/5.0
    Mozilla/5.0 (U; Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0

By the way, have you considered using this[1]? Seems the best option to prevent tracking.

    [1]: https://addons.mozilla.org/en-US/firefox/addon/randomuseragent/