This happens here: https://searchfox.org/mozilla-central/rev/952521e6164ddffa3f34bc8cfa5a81afc5b859c4/browser/components/about/AboutPrivateBrowsingHandler.jsm#23,33

We need to cache the bound functions to be able to unregister them, and line 33 doesn't even pass anything.

We should probably also change https://searchfox.org/mozilla-central/rev/662de518b1686c4769320d6b8825ce4864c4eda0/toolkit/components/remotepagemanager/RemotePageManagerParent.jsm#135 to throw an error if we didn't have the callback that was passed registered, or at the very least if nothing is passed.

Fortunately this is the parent process and on shutdown, so really the code could probably just be removed?