That isn't a Firefox issue but McAfee support to resolve for you. Also check with McAfee site and support about their software issue. Also Firefox Browser doesn't do popups to buy a product or software.

I would use process explorer to see what process is generating the pop up. Just use click the X in the menu bar, hold the mouse button, and then let go on the pop up. Process Explorer will then show you the parent process for it.

https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer

In my system lower right corner I get McAfee popup demands to be installed. How do I end this?

Are you using a OEM laptop or desktop from companies like Acer, Asus, Dell, Lenovo etc as they tend to come with extra junk like antivirus clients such as McAfee.

Neither the company Mozilla or the Firefox web browser has anything to do with McAfee in anyway as Firefox has never come bundled with anything McAfee.

If these are website notifications, you can get rid of them by removing notification permissions for all sites.