Safari 9 - I cannot install extensions

Glad to help out. Wish I knew what is the actual cause, but the safe boot work-around was acceptable.

THANK YOU!!!

Note that this stale certificate may also be in the "System" keychain (NOT System Roots) and cause this problem.

The root cause of this is a screw-up by Verisign/Symantec a decade ago, creating “VeriSign Class 3 Public Primary Certification Authority - G5” as an intermediate certificate signed by a weaker (1024-bit) root certificate. They "fixed" that error by re-issuing “VeriSign Class 3 Public Primary Certification Authority - G5” as a self-signed root with the same common name, distinguished name, and public key. That way they didn't need to re-issue all their customers' certificates signed with the intermediate. However, the removal of the stale weak root that had signed the original intermediate version from System Roots in El Capitan left the intermediate version as rootless, and it is actually indeterminate whether the system will pick the new root or the old broken intermediate when verifying a signature chain because all of the supposedly unique identifiers are the same. In principle the intermediate (in the login or System keychain) should be used but in testing chain verification in Keychain Access on a machine with both certs I found that sometimes a downstream cert used one path and sometimes it used the other. No theory why...

I will have to check into the certificates. I wouldn't think a safe boot would have any effect on the system handling them.

Thank you. This fixed the issue for me on yosimite + safari 9.

Thanks. This tip worked! Very weird that it matters if you "press" the button with keyboard or mouse.

Thank you SO MUCH! You saved me tons of my time with this simpliest hint! )))

I found this worked for me - this YT tutorial https://www.youtube.com/watch?v=IRWulGd1B18

I couldn't install the extensions before but doing his way worked.

Thanks! That worked.

That's really sketchy...

Thanks. After everything else this worked for me.

Open Keychain Access application, go to login keychain and remove all the expired certificates in there. Quit Safari and click on the icon of it while holding Shift button. This will launch Safari in safe mode. Go to extensions gallery and "install now" buttons should be there.