Try Start in Safe Mode, sometimes this problem is caused by addons:

• Firefox in safe mode

Case the problem doesnt occur in Safe Mode:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Sometimes the problem can be Malwares:

• Problems with malwares

Also, you can use addons to block popups/ads:

• Adblock Plus
• Simple Adblock
• Adblock Plus Pop-up Addon

Problem still exists in safe mode, scanned and no malware (though I'm already running MS Essentials), already running ad blocker. I'm not a programmer, but it seems to me that if the problem only occurs on certain pages and only in Firefox, not in IE, then it is a problem in the way Firefox is handling these files....Again, if I left click and switch rendering to IE (I'm running an IE tab, and have been for several years--have the most up to date version), this problem does not occur.

I accessed that web page. No adds, No popups. Everything was fine. Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

I already run MS Essentials and I did a complete scan--no malware.

I installed Ad Block Plus and it worked. Thanks.