Can you show us the pop-up?

Hmm, this could be a scam popup from a website that wants you to pay them for cleaning services. I'm going to re-post some old advice and you can let us know whether any of it helps:

There are a few common patterns to these annoying pages, and these are some techniques for closing them without having to take drastic measures.

The "key" (ha ha) is the keyboard shortcut for closing the current tab, which is Ctrl+w (or on Mac, Cmd+w). Try it after each action to see whether it is available yet.

(1) Large alert dialog (lots of text)

If you cancel this dialog, it may reappear. After two or three appearances, Firefox should add a checkbox at the bottom of the dialog to stop the site from showing more alerts. Check that box and click OK to block further dialogs.

(2) Authentication dialog (asks for username and password)

If you cancel this dialog, the page may reload and immediately show it again. Pressing the Esc key numerous times in a row can cancel the reload as well as the dialog.

(3) Reacting to mouse movement

Some pages have a script that detects when you are moving the mouse pointer up toward the tab bar and takes action to show another dialog, or moves to full screen view to hide the toolbar area. On these pages, the keyboard shortcut is essential.

Hopefully this will let you close problem pages without having to "kill" Firefox in the Windows Task Manager. (I don't recommend using that method because the tab will come back during automatic crash recovery anyway.)

Often these scam pages are promoted through ad networks. As a defensive measure, you could consider using an add-on that is effective at blocking ads, such as:

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

As with any content blocker, this will cause problems on some sites, so keep an eye on its toolbar button in case you need to make an exception to get a page to load properly.

Hello! Thank you for the advice ... None of the actions you suggested worked, but when I added ublock, the pop-up it disappeared. (I already have Adblock Plus). Thank you so much. I looked for a rouge App, or add-on in both Firefox and Edge to no avail but this ublock stopped it cold. Sorry I couldn't add a screenshot, but for some reason Firefox wouldn't let me.

Really appreciate your time and attention!

This happened to me, and I could not escape it at first! I closed Firefox through task manager, and opened a second Firefox Window, but each time I reopened it, the malware attack had the whole browser frozen on that tab. Ctrl + w did nothing. Adding ublock did nothing!

I decided to uninstall and reinstall firefox, and that's how I found a solution: In programs > uninstall or change, Firefox offered to "refresh" instead of uninstalling it, and that cleared the horrid malware!

Unfortunately, I was too startled to get a screenshot, but it had a print pop-up, and a dialog box pop-up, and wouldn't allow me to exit or escape in any way.

Anyway, I wanted to share my solution here: start to uninstall Firefox, but choose to refresh it from there instead of removing it.