This is caused by an extension. Possibly AVG or "eSnips Download Helper".

See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for how to disable all extensions, and enable them one by one to find the problematic one.

See also this thread: https://support.mozilla.com/en-US/forum/1/258251

Dear rjohnson19 - Thanks. I managed to stop the problem occurring. There were multiple extensions causing the problem. The culprit extensions also seemed to be throwing up Trojan warnings. I have disabled all of the bad extensions and run CCleaner. Does anything else need to be done to eliminate "hidden trojans"?

You can run and update the following free anti virus software:

1. Malwarebytes (Free version) - http://www.malwarebytes.org

2. Spybot Search & Destroy - http://www.safer-networking.org

3.Spyware Terminator - http://www.spywareterminator.com

In addition, there are a number of forums you can use to help get rid of your infection. These include:

• http://www.bleepingcomputer.com/forums/

• http://forums.spybot.info/

• http://www.spywarewarrior.com/index.php

Dear RJohnson 19 Thanks

I put Spybot on to protect