For extension install and use issues you should contact their support. And extensions don't require account to install from Firefox addon page.

Thanks Mark. I have raised the issue with Nord/Nordpass, was posting here just in case there was something on the Mozilla/FF side that made one extension accessible/viewable (?). In noting I was able to successfully install on same PC under another account, was referring to another Windows account (not FF account)... but that wasn't clear in my post. Will continue to pursue w Nord/Nordpass. Thanks again for the quick reply

The NordPass® Password Manager & Digital Vault extension page at AMO has been removed. https://addons.mozilla.org/firefox/addon/nordpass-password-management/

Two possible reasons are the NordVPN author removed it or the recent version violated extension policies as per https://extensionworkshop.com/documentation/publish/add-on-policies/