Some sites will open a "popunder" window, but if you notice a large number of windows on many different sites, you might have a bad add-on.

To clean up, I suggest starting here:

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. Take out as much trash as possible here.

Next, you can "mop up" many remaining issues with the scanning/cleaning tools suggested in this support article: Troubleshoot Firefox issues caused by malware. Those are free and take a long time to run. If they seem to finish quickly and especially if they require a purchase to clean your system, you may have a serious infection. Try the specialized forums listed in the article.

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

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

(1) In the left column, click Plugins. If you find a plugin that you don't recognize or which doesn't sound essential, try setting it to "Never Activate". (The most essential Plugin is Shockwave Flash. You can use most sites without the others.)

(2) 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.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for a good add / pop-up blocker.