The problem with disabling or not being able to install this addon is caused by a bug in Firefox https://discourse.mozilla.org/t/certificate-issue-causing-add-ons-to-be-disabled-or-fail-to-install/39047 Don't panic, it will be fixed soon
Hi there, Thank you for sharing your insight! You're correct. Firefox has also now pushed the fix and it should be installed for most users in the next couple of hours: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
