The short answer is that you have nothing to worry about because firefox doesn’t appear to have a clue as to how to get this to work in avast (thank god). So even with what was the default setting to scan when done it didn’t work with avast and many other AVs.

I disabled this when it first came out before I found out it didn’t work and I have had zero adverse effects. I haven’t touched my Windows Security Policy, but even so I don’t see how this would be effecting you as you aren’t “Launching applications and unsafe files set to Disable.” You are just downloading them to your system and not running or installing them directly.

Also if the browser.download.manager.skipWinSecurityPolicyChecks = false is no longer supported then does that entry still exist in about:config, it doesn’t in mine and if it isn’t there to check it shouldn’t be taking any notice of the IE settings. Mine are set to Prompt anyway, but this really has nothing to do with downloads as far as I can tell.