Yes, that looks like a fake site. The way you can tell is this:

Microsoft's real website is this:

https://www.microsoft.com

This website starts with this:

http://www.microsoft.com-maintenance-pc.live

Notice the difference? They are trying to make their address look like microsoft's, but failing.

So, yes, please ignore that. And then, beyond that, do the following to keep your computer safe:

• Download Malwarebytes from www.malwarebytes.com, install it, run a full scan, and remove anything it finds.
• Make sure you update Windows and install all updates it finds.
• Uninstall any anti-virus you have on your computer, and use Windows Defender

I was sure it was fake but didn't know how to confirm so thank you so much.I will do as you say but I already use Window Defender.

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.

Sometimes these scam popups spread through major ad networks on popular sites. If you haven't previously considering using an add-on to block ads, perhaps that would help. This one is well-regarded:

https://addons.mozilla.org/firefox/addon/ublock-origin/

Of course, any time you introduce a new kind of content blocking, there are sites that will not function properly. You may need to keep an eye on the extension's toolbar button and make exceptions from time to time for sites that can't run properly without ads and tracking.

Thank you.