I'm using this version and it has operated faultlessly.
I have found a post that says the user was using McAfee Anti-virus and he fixed the problem as follows;
"It is McAfee Anti virus that was causing my issue, If you excluded explorer.exe, ntdll.dll and any other dll's that are causing this error from the on access scanner it stops the error. I don't know if this will work in all cases but it did for me."
If your not using this, maybe try un-installing Firefox, check you have the most recent version, or if you have, download a fresh file and see if that cures the problem (just in case it's corrupted).