Add-ons are not installed

Yes moin … question: Recently several add-on’s have been deleted. If I want to install new, there is always “download failed, check the internet connection”. My connection is perfect but there is no download. Why is that?

Please make sure you’re on the latest Firefox version due to https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/