Does this issue occur on a certain website?

Firefox does not have popups telling users to upgrade to Windows 10. These are caused either by your operating system or a specific website (probably a Microsoft owned website).

Please try changing your homepage to a website that is not owned by Mircosoft, like Mozilla.org.

MSN (http://www.msn.com/) shows that message if you do not have Windows 10.

When I compare my MSN cookies before and after clicking the X to dismiss the message, I do see two additional cookies being added:

MUID=1CCCB2979E3F6D0C141BB5409A3F6B42; ANON=A=798F206669ED36B69AD6BDF9FFFFFFFF&E=115c&W=1; NAP=V=1.9&E=1102&C=9fAkBnkLwO94XK2ya3u8o9m_8wOnX9hqouddj4OBWHwDt-jvUiH9Zw&W=1; dpio=1.4; ipt={"v":{"L":1},"pt":{"d":1},"ct":{},"_t":49,"_v":"2"}; Sample=52; _SS=SID=00; pop=1; pbcls=1; popunder=yes; popundr=yes; setover18=1

Those seem to stop the message from displaying when I reload the page.

If I use the Show Cookies dialog to delete the pbcls cookie, then reload the page, instead of a popup I get a bar pushing down along the top edge of the page. When I "x" off the bar, the pbcls cookie gets re-added with a value of 1, and the value of pop is increased to 2.

I think the page checks for those cookies, to avoid bothering you on every page load. If you clear your MSN cookies, I suspect you'll get the pop-up style message again the next time you visit the page.

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