Up until today I have never had trouble with pop ups, but even though I use Firefox I get a pop up in IE. Does anyone have any ideas what it could be and how I can stop it.

I used to get them after visiting mucky sites.

Well Firefox and IE are two separate things. Firefox blocks pop-ups unless you authorise them, and IE doesn't. Just use Firefox and you won't have a problem.

I think he means an IE window is popping up despite him only using firefox.

Sounds like you have some kind of malware Fensy, update your antivirus and download a program called malwarebytes to scan your PC and remove any infection.

There's some nasty stuff going around at the minute, I got one recently that changed the pages i went to when clicking on google links and stopped me from updating my antivirus and editing the registry. Good luck!