Are you unable to download Firefox or install it?

If you can't install it, download a clean copy at https://www.mozilla.org/en-US/firefox/ then run it as Administrator (right click -> run as Admin)

If you can't download it, try clearing your temporary folder. (Ctrl + R opens the run command (type %temp%) then delete everything and try again.)

Was the download process of the installer interrupted? Try the full installer. Download and install this: https://download.mozilla.org/?product=firefox-36.0.4-SSL&os=win&lang=en-US

Try the offline full setup from www.mozilla.org/firefox/all

note the member who posted before me is not related to Sumo or Firefox Support.

edit: This was when the member had the misleading display name of Firefox Support before. The username may get changed too.

James said

note the member who posted before me is not related to Sumo or Firefox Support.

Right, I'm not related. Nobody on here is related to Mozilla or Sumo unless they're an adminstrator.