Install issue Signing could not be verified -260

[mikeMikeMIKE]

Trying to install NoScript into Firefox 3.6.13 on Ubuntu 10.04. I get the error

Firefox could not install the file at
http://releases.mozilla.org/pub/mozilla ... +fx+fn.xpi
because: Signing could not be verified
-260

Is this an SSL issue, or a file signing issue, or what? Any help would be appreciated.

Thanks in advance,

Mike

[Giorgio Maone]

You probably erased the UTN-USERFIRST-Object certificate authority from your cert store, or you've got a corrupted profile.