Hello,

If Firefox was downloaded from the official site, it should not show ads on its own. If you recently downloaded Firefox, did you download it from the official site?

• https://www.mozilla.org/en-US/firefox/new/

Can you provide a link to a site where you are seeing the ad? You can try installing adblocking extensions such as Adblock Plus to see if that causes it to go away.

You can also check if an add-on is causing this by running Firefox in Firefox Safe Mode. If running in safe mode causes those ads to disappear, please check your add-ons at:

• Tools (or ) > Add-ons > Extensions
• Remove any suspicious add-ons

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for more information.

Do a malware check with several 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:
  http://windows.microsoft.com/en-us/windows/using-defender
• 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
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Hello,

Thank you for your answer. Our computers are clean of malware. Our hosting provider after some scans tells us that the custom theme (that we are using for Firefox appearance) is responsible for the problem. Unfortunately we are not being able to restore the name of custom theme however you can see it at the attachment. The problem appears at the first use of site in the day and not for all sites. Do you have any similar reports?

Your screenshot shows that you use a lightweight theme (Persona) that styles the user interface area. Such a theme can't cause a banner to appear in the browsing area. Only an extension or malware can cause this.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can check the connection settings.

• Tools > Options > Advanced > Network : Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can

You can anyway still do the malware scans even if you believe that your computer is clean. That never hurts.

I have been experiencing the same problem i.e. a banner ad of a game called The Rise of Mythos appears every day (once only) upon loading my chosen webpage. Originally, I thought this was Malware that had some how crept into Firefox. However, I have just discovered that it is not limited to only Firefox. The banner also appears in IE. So the problem is not a Firefox problem persé

I have run all Antivirus programs known to me. All of the above plus Emsisoft and Eset.

I would welcome hearing from anyone who has a fix for this.