I think the ads are caused by some add-on you installed by accident, the ads are coming on Yahoo alone or on every website, test the below mentioned technique and report back what happened.

It is possible to attach screenshots and links in Support forums, check the bottom of the editor in the left side and above post reply button.

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
• On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

Thank you.

Yahoo Browser ??

Is that a 3rd party version of Firefox which is optimized for Yahoo?
IMO, that may install what many users feel is malware, and be the cause of your problems.

Please install the official version of Firefox which can be downloaded directly from Mozilla.
http://www.mozilla.com/en-US/firefox/all.html

the-edmeister thanks for the info even i forgot to notice that he said it was yahoo browser, i thought it was due to some add on.

While in Safe Mode, you can manually mark extensions as disabled, and you can extract a list if you have questions about individual extensions.

• Add-ons page: orange Firefox button (or Tools menu) > Add-ons > on the left side click Extensions

• Extension list: Help > Troubleshooting Information, button in the upper left corner labeled "Copy Text to Clipboard"

SO what they are sayign about reseting firedox is the right way to go about this but in the future I would recommend an add-on called ad-blocker it blocks all ads give it a try! on sites like:

• youtube
• facebook
• search results

Who mentioned anything about doing a reset for Firefox?

Did you do a malware check?

Do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.
All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked