Maybe you pasted it in the wrong console? You need the 'browser console' which is different from the one you open on random webpages.

Go to "about:config" (in the url bar), search for and enable devtools.chrome.enabled, then hit ctrl-shift-j (or you can open it from Menu -> Web Developer -> Browser Console).

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.

As for your other weirdness. Signatures are only checked once every 24 hours, and it's only been 20 since the cert expired, your other computer probably just hasn't re-verified the signatures yet. You can find some comments here about how to delay it if you want.

That's an expected error, it has to do with how that addon (which is non critical but published by mozilla and really just a part of firefox) is installed, once mozilla publishes a new version of firefox with an updated signing key it should fix itself.