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
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• 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

Did you see the reply cor-el posted, about checking your computer for malware?

See Troubleshoot Firefox issues caused by malware for more information.

Thanks for your interest. Yes I have been trying to use the programs cor-el suggested. The first one I tried to download was Malwarebytes but this seems to cost about £25 so I left it for now. Superantispyware and Spybot were more successful Both identified threats which I actioned to either remove or quarantine. The program MSER pointed out that I already use Security Essentials.

However the result is that today I still get the annoying popup when I start Mozilla. So I still have the problem!

John

All mentioned programs have free versions, only special (Pro) version require to be payed for.

Hello, i think the firefox pop-up blocker is enable, Firefox button > Options > Options > Content panel > Block pop-up windows [v].

thank you

Here is the MalwareBytes link copied from the Troubleshoot Firefox issues caused by malware article. This link makes it clearer that there's a free version (the "Download Now" link) and a "Pro" version that must be purchased.

http://www.malwarebytes.org/products/malwarebytes_free