When trying to "+ Add to Firefox", the following error message appears: "Download failed. Please check your connection." despite no connection issues whatsoever. (If there were connection issues, would I be able to write this review?)
The "connection" message is a generic guess, but most likely this was caused by the certificate expiration issue. See: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/ or https://www.jeffersonscher.com/ffu/armagadd-on_2_0.html
