Maybe a poor choice of wording but, new PC (Windows 10 Enterprise 64bit) from work and new installation of Firefox (50.1.0), essentially the exact image as on the previous PC down to the patching level. Install LastPass universal installer to set it up on IE, Chrome, and Firefox. Works fine on IE and Chrome, but while FF completes I cannot log in through the plugin and it remains greyed out. Clicking on it "depresses" it, but nothing else.
I am extremely limited in what I can change on my laptop due to corporate security in place, so I can't make major changes. I've removed the plugin, uninstalled and reinstalled Firefox, turned it off and on again (Yes, I've seen and lived the IT Crowd). But on a different laptop with a nearly identical Windows image on it (HP vs Dell), it worked.