What is the second device, also what version is the second device on?

Please first try to disconnect the account, and log in. You should receive an email confirmation about the second login if you are on an updated version of Firefox.

Second device is Windows 7 Pro using a fresh install of the current Firefox.

Unclear what's meant by "disconnect" the account. If you mean logoff/on from the original device and/or just logoff the original device and login to second device didn't work either.

I am requesting help, however to double check, both devices are on the most recent version of Firefox?

Yes both computer are Win 7 and using the same version of Firefox 49.0b4. Also tried 49 on the second computer: no help.

Any particular reason that you are using the pre-release Beta version?

Firefox 48 is the current Release version.

Maybe there's a problem with Sync and the current 49.0beta4 version?

Given your implication is that no one ought be using the beta versions is a silly condescending question.

Release 48 also results in sync password failure. And yes removed and replaced with version 48 Firefox on the second computer. Also reset the password on the primary computer again and after downgrading to 48.

It was just a question, no need to get your panties in a bunch!