Every time I install an upgrade to Firefox, the main page continues to tell me to upgrade to a latest version, because it thinks I'm still using an outdated version. This happens with other sites that are no longer supporting older versions, even though I'm up to date - they're just not recognising it. E.g. YouTube continues to display that message at the top of the page asking me to update my browser, even though it is the latest version. This all happens with sites where Flash Player is needed, thinking that I'm using an outdated version of that, too, even though I'm not. How do I get the browser to recognise that I'm using the latest version?