Since updating to v18 it's there every time I close down Firefox, in a separate window. Please how do I get rid of it for good?
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 Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.
Further information can be found in the Troubleshoot Firefox issues caused by malware article.
Did this fix your problems? Please report back to us!