My husband got that once.

Usually, just a reboot is fine. If you do shut down, there's no reason to leave it off for an hour - nothing is going to happen in that hour beyond the first 15 seconds or so. That's long enough to reset most hardware.

The fake alert windows can also often be closed simply by pressing CTRL + F4.

Turning it off for a few seconds didn't work.

What happens is that malicious websites will hijack your browser to pop up warnings that take you to a non-MacAfee site that aims to hijack your money. They may ask you to download their "Antivirus" product, which ITSELF infects your computer.

These articles discuss how to deal with this, in degrees of sophistication,

https://www.msn.com/en-us/news/technology/what-is-the-mcafee-virus-pop-up-scam-how-to-get-rid-of-it/ar-AA14b42z

More detailed.
https://malwaretips.com/blogs/remove-fake-mcafee-pop-up-virus/.

I haven't vetted sites to see if their aim is to sell you their own antivirus product, but the base malwarebytes app is free (just not the advanced features) Some sites will give you the basic information, and then say "Well, it's SO MUCH easier when you buy our product"

If your system seems clean, then
The goal is to clean out any garbage that got into your browser's memory.

Asking others to comment if they have dealt with this.

I think your browser needs a hard reset. Just a guess.
Check parts of those articles that address the browser.

There are bits of history and saved data that are in your "profile" (home directory, whatever your system) that would come back even if you reinstalled the browser. So, do what you can to clear all history and data and even maybe extensions.

Yeah, I had a tiring day, myself.

I'll bet that if you installed another browser, just for fun, it would not have the problem, and there's your Sherlock clue.

Good luck.

If that doesn't work, restart your PC.

It's probably just an infected webpage. In the event that it downloaded and installed malware, it's best to run a scan.

If the problem doesn't go away after a reboot, download and install the free version of Malwarebytes. Run a scan and let it remove whatever it finds.

https://www.malwarebytes.com/

For future protection, download and install their free Browser Guard: https://www.malwarebytes.com/browserguard

Note: it will be detected as an ad blocker and can cause some issues with sites that have paywalls. The ad protection portion can be turned off for individual sites.