Extra "pop-under" windows can be generated by websites, but if these are random and unrelated to the sites you're visiting, I would suspect a bad add-on.

Since those often are installed externally to Firefox, I suggest starting here:

Open the Windows Control Panel, Uninstall a Program. Click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

Then, in Firefox, open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Finally, you can "mop up" remaining issues with the scanning/cleaning tools suggested in this support article: Troubleshoot Firefox issues caused by malware.

Does that clear it up?

1. Start Firefox in safe mode to check if one of the extensions is causing the problem:
   • Firefox in safe mode
   • See Troubleshooting extensions and themes if the problem doesnt occur in safe mode.
2. Malwares can also cause these problems:
   • Problems with malwares
3. You can also use some add-ons to block ads/popups:
   • Adblock Plus
   • Simple Adblock
   • Adblock Plus Pop-up Addon