Tracking bug for signing all OS X builds. Because certain features, like keychain integration, require signed builds we'll have to sign all builds: continuous integration ones, nightlies, and releases. Like with Windows, we'll use our self signed key for CI builds and nightlies on certain branches, and use different keys for the other nightlies and releases.
*** Bug 752613 has been marked as a duplicate of this bug. ***
As far as I'm concerned, this is FIXED. Bugs 758098 and 758188 are tracking non-blocking follow-up issues, but Nightly, Aurora, and Beta (as of 13.0b6) are now signed. 13.0 will be signed. Everything else (project branches, dep builds, try, etc.) are signed with our self-signed key.