Scan for Malware

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 some of the following programs to scan for malware:

• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Microsoft Security Essentials (A good permanent anti-virus if you don't already have one)

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

Reset Firefox

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Thanks for this advice which I have followed successfully. The virus was alerted by AVG, with which I also scanned the C drive. The virus was moved by AVG into the virus vault, where I deleted it, but it repeatedly returned whenever I used Firefox. AVG identified the virus as residing in a cache file in a folder 'adblock', in the Firefox profile . Resetting Firefox created a new profile, which did not contain an 'adblock' folder. The virus has not returned. I have deleted the earlier profile. I do not know why 'adblock' was in the original profile, but you might well investigate this and take steps to prevent such an occurrence in future.

Thanks again.

Adblock Plus is a Firefox extension to block content on web pages.
This may have been a false alert.

• Adblock Plus: https://addons.mozilla.org/firefox/addon/adblock-plus/
• http://adblockplus.org/en/getting_started

Upload the file that avg claims it is a virus to a site like https://www.virustotal.com/

Also did you do any updates in avg recently as sometimes a recent definitions update can result in false positives on some Firefox related stuff and then fixed in a new definition update.

Thanks

Cannot do that - the file has been deleted and has not re-appeared.

I have the latest version of AVG & Firefox. As adblock was not reinstated after doing the reset which you advised, AVG does not have the opportunity to detect any possible associated virus.