I use mozilla firexox myself and have no file/folder there called runner..
What version of Firefox do you use? Mine is 3.5.2.. Perhaps if possible you should send the file to virustotal.com and see what the other scanners thinks about it..

My version is also 3.5.2
It has finished scanning and reported 5 files infected. Here they are;
TrojWare.Win32.Trojan.Agent.~(ID = 0x659b9c) C:\Users\*name\AppData\Local\Temp\Temp1_Runner.zip\Runner\lt-runner.exe
TrojWare.Win32.Trojan.Agent.~(ID = 0x659b9c) C:\Users\*name\Downloads\Runner\Runner\lt-runner.exe
TrojWare.Win32.Trojan.Agent.~(ID = 0x659b9c) C:\Users\*name\Downloads\Runner.zip:Runner/lt-runner.exe
Application.Win32.LeakTest.~LT(ID = 0x90b95f) C:\Program Files\COMODO\COMODO Internet Security\Quarantine\LeakTest.exe:UPX
TrojWare.Win32.Trojan.Agent.~(ID = 0x659b9c) C:\Program Files\Mozilla Firefox\runnerwhat do you think? delete?? report???