I have not used McAfee for a long time and I don't know exactly which product or version of McAfee you have, but I suspect it may have to do with the "wait until my PC is idle" setting somewhere in McAfee?
Having said that, running an old, outdated release version of Firefox creates significant security vulnerability.
I presume you are doing this because you want to use an unsigned extension that is being blocked by current Fx versions (48.0.2).
There's no love lost between Mozilla and me. And the extension signing situation has been problematic even for some legitimate extensions.
But continuing to run outdated versions of Firefox with outdated, unsigned extensions is not a safe long-term strategy.
A safer solution to your problem might be to run Firefox ESR:
I think the current ESR version is 45.0.2:
ESR gets security patches.
Just a thought,