I can download Mozilla firefox, but it will not install for some reason

I am using Windows Vista Home. I have downloaded the app for Firefox several times without a problem, but nothing pops up or opens for me to install it. I had previously uninstalled Firefox and was going to re-install it, because it was acting up (running slow), thinking this would help, but like I said it will not give me avenue to re-install it. I would really appreciate some help asap, due to the fact that this is the browser that I do all of my home business with.

Thanks in advance, SShep