Firefox should not display popups about Windows 10. This is either your operating system or a website giving you this popup. Some Microsoft websites will contain this popup because they are trying to get people to switch the Windows 10. Please try changing your homepage to a different website for the options tab. For steps on how to do this, please view with Mozilla support article: *[[How to set the home page]] If changing your homepage to a website that is not related to Microsoft doesn't fix the issue, it is possible that you have malware on your system that is trying to get you to install an infected version of Windows 10. To find out to fix issues caused by malware or other infections, please refer to this Mozilla support article: *[[Troubleshoot Firefox issues caused by malware]] Please let us know if this fixes your issue.