Firefox doesn't store files in that location. It may have been the result of some download action.
If that file is still in the %temp %folder then you can remove the file.
You may need to boot the computer in Safe mode by pressing the F8 key on the boot screen.

Do a malware check with a few malware scan programs.
You need to use all programs because each detects different malware.
Make sure that you update each program to get the latest version of the database before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page

See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website

Thanks Cor-el - bear with me but do I start the pc up in safe mode and then delete the file or t'other way round? Also when I looked at the Microsoft Help Centre to find out about starting up in safe mode it doesn't explain what it means by the 'basic files and drivers' that I can access so I'm not clear what I'd be able to do in this mode.

If you can't remove that file in regular mode then you need to boot in Safe mode to see if that allows to remove that file. You do not need to load extra drivers or network support. After you have removed the file then you can reboot again in normal mode with all drivers and services running.

Thanks for your help Cor-el ; I was able to delete the file as 'normal' , but I'll keep your advice against future problems!