There is an issue with Firefox that prevents Add-ons from being installed/running. Mozilla is working to resolve this issue.
As per their blog: "Late on Friday May 3rd, we became aware of an issue with Firefox that prevented existing and new add-ons from running or being installed. We are very sorry for the inconvenience caused to people who use Firefox"
You may read about it here - https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
