Are they double-underlined? (Edit: Oops, I see you said they are.) You probably have installed an extension that links to an advertising service. Try disabling ALL nonessential and ALL unrecognized add-ons on the following page. Don't be fooled by bland names such as "Browser Manager"; most extensions aren't necessary for normal browsing.

orange Firefox button (or Tools menu) > Add-ons > Extensions category

After restarting Firefox, do the ads go away?

See also http://botcrawl.com/how-to-remove-text-enhance/

You can also do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.

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
• 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.

• http://support.kaspersky.com/viruses/solutions?qid=208280684

See also:

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

Solved! Thank you very much.