There's already been a howl from the security world about inviting security breaches. If you allow Firefox (who _is_ that?) to install things on your computer when you're not watching, who else might like to do that? I do believe this is a Windows problem because Linux or BSD will not allow security breaches of that kind. My guess is that you can turn it off in Windows if you want. It would certainly be a security nightmare for administrators of larger systems if you couldn't.
As for old versions. I installed FF 1.5 recently on some old kit. Version 3.6 is still a good choice. Get that here. If you want old versions of many other browsers, including FF, get those here.