Ah, yeah... very difficult to understand, indeed.
There might be something wrong with your previous Firefox installation. Completely remove/uninstall Firefox as well as manually remove the installation folder in the Program Files and in the profile folder. Since you're using Windows 7, after Firefox uninstallation, go to your windows user profile folder and delete any Firefox remnants there.
It should be at this directory; C:\Users\<username>\AppData\Local\Mozilla\Firefox