This are common scams. I suggest installing something like https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/ to block ads.

By the way, there are a few common patterns to these annoying pages.

(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. On these pages, try using the keyboard shortcut Ctrl+w to close the tab instead of the mouse. (On Mac, Command+w)

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.)

If you see this even though are not visiting the same site, one possible culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions.

Then cast a critical eye over the list on the right side. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove).

Any improvement?

So I added Ublock. I do think the popup comes up at different sites but I'm not sure. I don't see anything unusual in my extensions, there isn't much there.

Please scan with eset online scanner : www.eset.com/Online/Scan‎ and with the Portable https://www.adlice.com/download/roguekiller and with https://www.malwarebytes.com/adwcleaner/

If need help please see the pros : https://forums.malwarebytes.com/topic/9573-im-infected-what-do-i-do-now/

Please let us know if this solved your issue or if need further assistance.