Your UA says it's Vista, not 7.

If it's 7, you should able to install it if you have 2GB RAM or more.

No, it is Windows 7 Home Premium 64bit. I just ran the Windows Update to be sure there wasn't anything missing and it loaded Win7 updates. If I had "moved" my software and files from my old machine (windows vista) several years ago, would Firefox still think it on a Vista machine? If so, how to get rid of the old version?

First, could you check why your Firefox tells websites you run Vista? The most common reason would be using a compatibility downgrade. You can check that as follows:

Find the icon you use the start Firefox, and either:

• Right-click a desktop shortcut, then click Properties
• Right-click a Firefox icon pinned to the Taskbar then right-click Mozilla Firefox, then click Properties

You should clear all the boxes on the Compatibility tab for best results (giant screenshot below). Then exit/restart Firefox to check the results. For example, on my test page, it should say Windows NT 6.1 instead of Windows NT 6.0 in the "user agent":

https://www.jeffersonscher.com/res/jstest.php

By manual install are you using the full Firefox 57 setup for 64-bit Windows from www.mozilla.org/firefox/all ? or the stub installer from www.mozilla.org ?

Your user agent shows it as Firefox on Vista (NT 6.0) : Mozilla/5.0 (Windows NT 6.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0

https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

Though it could be you are running Firefox in Vista compatibility as mentioned above.

Thanks to everyone for all the help. For some reason it was running in "compatibility" mode. Eliminating that allowed it to update to Quantum.