When I try to install any app I get to the message
Firefox could not install the file at
https …
because: signing could not be verified.
-260
How can get the app?
Are you talking about apps or add-ons? Which one exactly? Please provide a link.
Sorry, it is add-on.
The add-on is https://addons.mozilla.org/en-US/firefox/addon/1-click-youtube-video-downl/?src=ss
but it happens with some other that I have tried.
Works for me just fine. Please try again.