hello BillCatz, this might be an issue with malicious addons or adware already present on the system. please perform all these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

philipp said

hello BillCatz, this might be an issue with malicious addons or adware already present on the system. please perform all these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

I have done that but popunders keep showing up from various sites. I just reran a fresh version of MalWarebitese and Spybot plus my AVG and all came up clean. As I originally noted, other browsers block these so I don't understand why Firefox can't.

FredMcD said

Adblock Plus {web link} Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).

Adblock blocks only a fraction of the ads. Most still get through and it does nothing for the pop-unders.

Adblock blocks only a fraction of the ads. Most still get through and it does nothing for the pop-unders.

All of my pop-ups are trapped and gone.

Adblock Plus Pop-up Addon {web link} Adblock Plus Pop-up Addon extends the blocking functionality of Adblock Plus to those annoying pop-up windows that open on mouse clicks and other user actions.

Element Hiding Helper for Adblock Plus {web link} Fight the text ads! Element Hiding Helper is a companion extension for Adblock Plus meant to make creating element hiding rules easier.

No adware blocker seems to address the popunders. Why is that? If you can block popup ads with the browser then why not the popunders? They render the Firefox browser useless... Either by freezing or crashing.

A popup window and a popunder window are exactly the same except that with a popunder, the site cleverly gives the focus back to the active window so you aren't infuriated immediately.

Firefox allows popup/popunder windows for certain user initiated events. If you want these events to be handled as suspicious, you can change a hidden setting here:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste pop and pause while the list is filtered

(3) Double-click the dom.popup_allowed_events preference and delete its contents -- or remove selected events if you know which ones sites are abusing -- and click OK. (If you want to start editing from scratch, you can right-click > Reset to restore the default list.)

I don't know about other sites, but this works on SeatGuru which normally opens a popunder when you click the name of an airline. (You'll briefly see the popup notification bar before the airline page loads.)