In some cases, this might be caused by that same site finding you browsing elsewhere and trying to get you back. Sometimes when you leave a site, it will generate a pop-under window. This can be hard to spot, as Firefox might just briefly border the icon on the Windows Taskbar. You can click the Taskbar button to see whether a new window appears at the bottom after leaving the site. Does that seem to be the pattern?

If the ad is for a different store, however, I would suspect a bad add-on. Here's my suggested procedure for tracking down and cleaning up bad add-ons and other hijackers. I know it seems long, but it's not that bad.

(A) Open the Windows Control Panel, Uninstall a Program. After the list loads, 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. Be suspicious of everything you do not recognize/remember, as malware often uses important or innocent sounding names to discourage you from removing it. Take out as much trash as possible here.

(B) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions. Bear in mind that all extensions are optional, none come with Firefox.

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.

Any improvement?

(C) You can search for remaining issues with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

Success?

When I open the Control Panel, where does it say "Uninstall a Program."

ChaCha said

When I open the Control Panel, where does it say "Uninstall a Program."

On Windows 7, it is in the Programs category. If Windows 8 doesn't have it organized that way, you could search for Uninstall a Program in the "charms bar" search box.

I have Windows Vista. Where would I find it.

Sorry, I don't know why I thought you had Windows 8. I never used Vista myself, but if you cannot find Uninstall a Program using the category view, you should be able to change the view to show all of the items in a list.

You also could try pasting the following in the Start menu search box and pressing Enter:

appwiz.cpl

It says to select a program, and then click Uninstall. What program am I supposed to select so I can uninstall it?

Hi ChaCha, sort the list by install date by clicking the "Installed on" column heading after the list loads. This should bring the most recent additions to the top. If the oldest come to the top, click again to sort in the reverse order.

Look at what is listed. Do you remember installing those programs? If not, they may be unwanted software.

If you aren't sure, do you want to list what you see there having been installed recently? It would be useful to know the program name and publisher and version. Or you could post a screen shot of the list. This article describes how to grab a screen shot: How do I create a screenshot of my problem?

I typed up the words "appwicz.cpl" and hit enter in the Start area. Then all of a sudden my monitor went dark and my computer changed colors from blue to orange and so did my moniter light. Then, I was not able to type anything because it seemed like my keyboard froze up. What was that all about? I had to shut off the computer and then unplug the keyboard cord for a few seconds.

Windows might display a User Account Control dialog to access the Uninstall a Program control panel, and that might involve a dark background behind the dialog, but I haven't heard of Windows going haywire that way. Then again, I never used Vista.

Could you try starting Windows in Windows Safe Mode and see whether you can access the dialog in that mode? Usually when you're starting your computer and you get an initial black screen, tapping the F8 key repeatedly will give you a list of startup options, one of which will be Safe Mode with Networking. And/or see: http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/#vista

I just installed Adblock Plus. I will see if that helps with those pop-ups.