Are you using the official site such as getfirefox.com

Obviously if downloads do not complete there is no way that Firefox will install correctly. The installers from about:firefox start as stubs. The full exe installers are available from https://www.mozilla.org/firefox/all/

• See How to install Firefox on Windows
• Are you getting any error messages on screen, or can you find any in the Windows event viewer.
  • http://windows.microsoft.com/en-gb/windows/open-event-viewer#1TC=windows-7
• You may need to check settings of security software or even turn it off (Any XP users see additional small print under)

_{(There is a glitch with Windows XP {See Firefox installer crashes on Windows XP } and Firefox 35 at present and the firewall will need to be left on, but this question appears to be related to Windows 7, per the system information aside.)}

You rock ! Thank you.

I tried getfirefox.com - it supplied the stub option - and I got the 'your download was interrupted' message I was expecting.

So I tried https://www.mozilla.org/firefox/all/ - and it worked fine

Just to confirm... you rox like a firefox (:

Glad it worked. Thanks for posting back.

Not sure if there is or was a temporary problem. I do not seem to be seeing many others reporting the problem, but it worked for you, that's the important result.