Could you try using the "full" installer from the following page (scroll down to your preferred language):

https://www.mozilla.org/firefox/all/

I suggest saving the file to your Downloads folder, and then running it from there.

Any improvement?

Kathyjay1710 said

I've had Firefox before. I uninstalled and now I can't install it. I hit 'run', but nothing happens.

I ran it from the full Firefox. Tried three times. Each time it got to 'scanning security', it stopped. Just dropped me.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) to see if that helps.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/