Firefox 48 no longer allows any exceptions to the verification requirement.

Is it an extension for which there is an updated version or a replacement, or is only an old version available and no other good options?

This article lists some options for advanced users: Add-on signing in Firefox.

It is my PC and I and noone else decide what is running on it! If i want to use this addon, it is a good idea that firefox gives a warning to me, but there MUST be the possibility to use that addonn nevertheless!!! (e.g. via xpinstall.signatures.required)

If this will not possible in nearest future i will no longer use firfox! This paternalism is an absolutly NOGO!

See this: http://www.largrizzly.net/firefox.html

Hardy74 said

It is my PC and I and noone else decide what is running on it!

You must be using a pretty old extension or it is not being hosted at addons.mozilla.org by the author for it to not be signed.

The workaround posted by the-edmeister http://www.largrizzly.net/firefox.html solved my problem.

Thank you the-edmeister! :-)

I hope mozilla will fix this intolerable paternalism soon!

Hardy74 said

I hope mozilla will fix this intolerable paternalism soon!

Don't hold your breath waiting for that to happen.