Because when the FireFox team gets the latest version out for security fixes, they aren't "official" releases. It's recommended to uninstall FireFox, before installing the latest security fixes, otherwise, multiple versions show up in add/remove programs.
I've just gotten used to it as I've never bothered uninstalling FireFox before installing a new version. As long as you install to the same location, it doesn't take up any more space, just looks odd when going through add/remove programs.
However, I did find out the hard way, that if you remove the older version from add/remove programs... it removes Firefox entirely, since it's pointing to the same folder/file. So either uninstall before updating to a new version, to clear out add/remove programs, or just live with the multiple versions showing up.
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