Firefox applies the same rule to pop-ups and pop-ups that hide behind other windows (pop-unders). If there was a user-initiated action such as a click then the site is allowed to open a window.

Is the problem with the pop-under that it is the last window, so closing it closes Firefox, or that the page in the window is somehow able to close ALL your open Firefox windows?

Hi, the Pop-Under can close the whole browser. That's what I mean when I say, the Pop-Under is not like an ordinary Pop-Up; it seems to hi-jack the whole browser and become the main window.

What is the web address? What is on the pop?

It could be the work of one of your add-ons, or even add / mal-ware. Look thru your add-ons list and make sure you know what each one is and what it does. Also, check the programs that are on your computer

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the "Applications" folder Linux:

• Ubuntu Unity {web link}
• Xfce: Applications Menu category sections
• options depends on the package manager and the desktop environment

Go thru the list. If you find something that you don't know what it is, use a web search.

Troubleshoot Firefox Issues Caused By Malware {web link}