I'm being threatened by email with closure of my account unless I sign in by clicking a button in the message.

https://support.mozilla.org/en-US/kb/mozilla-account-deletion-inactivity If this was about your Mozilla Account then this may be legitimate emails warning you to login to your Mozilla Account before a specified date or it will be deactivated due to inactivity for two or more years. It is possible to use the Firefox web browsers without using a Mozilla Account for anything or infrequently. This shows what a Mozilla Account is used for https://support.mozilla.org/en-US/kb/access-mozilla-services-firefox-account#w_what-services-can-i-use-with-a-mozilla-account If this old Mozilla Account you used to post this thread is the same one with the warning emails then it should be considered active now.