+++ This bug was initially created as a clone of Bug #914225 +++ Bug 914225 has the fix to support the hot-fixing for thunderbird, but to ensure that it works we need a signed hot-fix add-on. Attached to that bug is the add-on we want signed (attachment 8432130 [details])
Signed and uploaded to bugzilla as attachment 8434565 [details].
Ugh, unfortunately the xpi had a version problem in the install.rdf (which prevents uploading to amo, but doesn't prevent running it), so we need to sign an updated version, which I attached it as attachment 8443988 [details].
Done, attachment 8444850 [details].