This is a restartless JS addon that overrides the default way Firefox deals with
GMail's S/MIME attachments. It works well as long as the following conditions
are met:

  1. Your secure Bugzilla mail is sent to GMail.
  2. You have an S/MIME cert configured properly in Bugzilla.
  3. You have the same S/MIME cert configured properly in Firefox.
  4. You're trying to decrypt a simple secure email message generated by
     Bugzilla.

All you have to do is open a secure Bugzilla message that's marked with an
"smime.p7m" attachment. Click the attachment and the view-source window will
appear with the decrypted message text.

That's it.