Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one. Windows 8 has antivirus built-in already.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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miserablesod said

I started to get all sorts of pop-ups wgich seem to com from A.adsmktng. I did a boot scan with Avast which found no PUPs but still had it. Ran a search for the string 'A.adsmkt' which was found only in firefox.exe. I refreshed Firefox - the string was still found , but in OLD FIREFOX/firefox.exe and 3 other files in OLD FIREFOX/ but still had it. Since I have IE8 but never use it, I uninstalled and then re-installed Firefox (worrying thing is that Bookmatks remained) . Ran firefox again and its still there - I typed the address rather than use the bookmarks of which Im highly suspiciouis , since I expected them to be deleted as part of the uninstall. If I run IE8 I have no problems and have actually visited a couple of sites which have the pop-ups if I go there with Firefox but with IE8 I get a 'clean' page. The problem therefore is within Firefox, so how do I get rid of this coz I really dont want to be running IE8 - I'd far rather continue with Firefox Running under XP sp3

I HAVE managed to cure this. I did a search for the string 'A.admrkt' and found that it had been embedded into firefox.exe. I took your advice and ran the adware stuff which required a reboot and it found stuff I didnt know I had (expected Avast! to have run interference on them)' but the A.admkrtng stuff was still there - not just marketing stuff but also some spurious and very suspicious stuff for adware removal programs and PC Optimising programs. I decided that it was time to get the sledgehammer out. I removed anything with a reference to Mozilla on the premise that if the code had embedded itself into the exe, it may well have embedded itself elsewhere in order to self-propagate . I then emptied the recycle bin, rebooted and did a fresh download and install. It has cured the problem but its a bit of a bugger that I will now have to rebuild my bookmarks menu and stuff but at least that bloody marketing stuff has gone ! Caveat : I picked up the infection whilst using the free wifi at a hotel I was staying at - I didnt download anythig - only browsed - so surmise that I must have picked it up when registering with the hotel widi