The hotfix extension does two things:
1) Install a new certificate for "CN=signingca1.addons.mozilla.org/emailAddressfirstname.lastname@example.org", effectively replacing the old certificate that expired. This should work.
2) Then it tries to import the internal "resource://gre/modules/addons/XPIDatabase.jsm" module and calls XPIDatabase.verifySignatures().
This does not work on ESR, as "XPIDatabase.jsm" is a new-ish thing that isn't present in ESR yet. In ESR the function is still in "resource://gre/modules/addons/XPIProvider.jsm" (XPIProvider.verifySignatures()). Thankfully, the non-existing module is imported using ChromeUtils.defineModuleGetter, which only lazily loads the module on first of the imported property, so after the certificate-adding code has run.