Seems that it will not work in the new version of Firefox :-(. But necessary for me, because the terror of nonsensical cookie warning is unbearable. Seems to be a bug in Firefox or result of the terrible Updates of Firefox :-(. Maybe Chrome?
Firefox has some problems with certificates which might prevent extensions from being installed (at least on Android).
Announced here: https://discourse.mozilla.org/c/add-ons Explained here: https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/ News about the issue: https://twitter.com/mozamo
Temporary solution: 1. go to about:config (enter in address bar of firefox), search for: "xpinstall.signatures.required", set it to "false", restart Firefox. Now you can install extensions again. Make sure to enable signatures again when the problem is fixed.