It is usually sufficient to create a new profile for the other user.

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Bypassing_the_Profile_Manager

Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

I was asking about how to have multiple Windows user accounts, and how to install Firefox independently for different Windows user accounts.

Firefox does not ask if you want to install it for everyone or just the current user, unlike many programs.

I tried switching to the newly created Windows user account, which was able to use the Firefox installed by the other user account, and installing Firefox, and although there's no indication that it's installing a user specific copy of Firefox, it is. Now the original user account sees it's installation of Firefox and the second user account sees it's installation of Firefox and no longer runs the first users installation of Firefox.

Creating separate Firefox profiles is what I was trying to say I didn't want to do. That's why I mentioned wanting completely separate copies of the program files, not one copy of the program files running two instances simultaneously.