Ha! That did the trick! Though it gave me these cryptic errors the add-ons are now enabled, thanks!

TypeError: setting is undefined[Learn More] ExtensionPreferencesManager.jsm:90:7 No matching message handler for the given recipient. MessageChannel.jsm:924 1556983316679 addons.xpi-utils WARN Add-on fxmonitor@mozilla.org is not correctly signed.

Edit: another weirdness: i decided to take a look at a different computer (unrelated to this one, at work via remote desktop) which is running exactly the same FF and Windows and all add-on are enabled. it's on 24x7; i didn't do anything to it.