FF nor Windows does popups. This would mean you got malware infections and that goes beyond FF user help here.

B1O1 said

why do you need to make changes to my computer i don't like pop up at start of opening window how do i make it stop ! pretty simple it was not there before update now it keeps coming up whats the go!!

What does the pop-up say? If it's too much to type, could you share a screenshot? This article has suggestions for capturing screenshots: How do I create a screenshot of my problem? Please cut out or blur any sensitive information in the image before attaching it to your reply.

If your Firefox shortcut is set to run as "Administrator," that can cause a message about Firefox making changes to your computer. This message is from Windows, and not from Firefox. To stop it, turn off the Run as Administrator setting. I don't know about Windows 10, but on Windows 7, you can find that by calling up the Properties for your Firefox shortcut, going to the Compatibility tab, and unchecking a box there.