That is certainly out of normal since in normal scenario, Firefox should have it's executables under program files and it's data in appdata/roaming/mozilla
Based on that (and the fact you have never had firefox) I would call it a remnant of somekind of virus/malware attempting to mask as firefox that your virus scanner possibly killed off automatically. (hence why new scans wont find anything, possibly)
all that was left is the batch file/run at start registry entry pointing to a file that no longer exists.
possible places to check for the running command on that non-existing file (to get rid of the error)
check if said Mozilla folder exists in said place, if it does, consider deleting it since it should not be there.
check startup on msconfig
check registry on regedit: These should be visible on msconfig but... you never know.
check HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
check HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce
check HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
check HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce
check start menu startup folder
check task scheduler, it might be timed task too.