Required add-on signing for extensions is new in Firefox version 43. Back in February 2015 [https://blog.mozilla.org/addons/2015/02/10/extension-signing-safer-experience/ it was announced] that, for a safer add-on experience, all extensions will have to be verified and signed by Mozilla before they can be installed. More insight into that decision is contained in Mozilla's [https://blog.mozilla.org/addons/2015/04/15/the-case-for-extension-signing/ April 2015 blog post]. Extensions hosted on [https://addons.mozilla.org/ addons.mozilla.org] are automatically signed. If an extension you wish to use is distributed by a third-party, it must be signed before you can install it. Unsigned extensions that you have already installed will be automatically disabled. Please ask the add-on developer or vendor if an updated and signed version of the add-on is available. You can also visit [https://addons.mozilla.org/ addons.mozilla.org] to see if there's a signed version of the add-on you can install. This signing requirement is mandatory in regular release and beta versions of Firefox. For testing purposes, it will still be possible to configure Firefox Developer Edition and Nightly versions to run unsigned extensions. For more information, please see [https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox Add-on signing in Firefox] and [https://wiki.mozilla.org/Add-ons/Extension_Signing Add-on extension signing]. You can visit the [https://groups.google.com/forum/#!forum/mozilla.addons.user-experience mozilla.addons.user-experience] group to further discuss add-on signing.

Thanks for the answer. But i need help. i am not an expert in stuff like this. I t sounds very complicated for me. So I must use CHROME (and i don´t like that at all)

Did you read all the hyperlinks that were embedded in christ1 posting? The "fix" is here: https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-can-i-do-if-firefox-disables-an-installed-unsigned-add-on