Before Safari 9, I was able to install any extension on Safari without any problems, but after Apple updated its Developer Program, it seems that I can't install extensions not already signed by Apple and in Safari Extension Gallery. I had two untrusted extensions installed and working, but had to remove those and I can't install them again.
I'm still having hope, because when I double click the extension file, Safari shows me a dialog like this:
But nothing happens when I click on
Trust. It just dismisses the dialog and won't install the extension. Is there any way I can install this extension?