Not authorized? Hmm... do you run Firefox as Administrator? That's generally not recommended.

This article explains why you get offered an old version if you try to download using XP or Vista, and how to work around it: How can I get the latest version of Firefox on Windows XP or Vista?

(Actually it doesn't explain it much. What happened is that Microsoft now requires a higher grade digital signature that Windows XP before SP3 and Windows Vista without an update cannot handle. Since the download site cannot detect your service pack and patch level, ALL Windows XP and Vista users get the last compatible version. Possibly in the future there will be a way to solve that, but not at the moment.)

Windows will not install the KB2763674 update to fix the signature problem, says it does not apply to my computer. I don't really care if I get any more updates I just want to stop getting popup warnings 10 times a day!

Did you follow the link in the article to get Firefox 45 -- it's under "Advanced Users"? For quick reference, open this page and click into your preferred language. For example, U.S. English is en-US. If you don't see a relevant code there, let us know what you're looking for.

https://download-installer.cdn.mozilla.net/pub/firefox/releases/45.0.1/win32/