This is a downstream change introduced in [1]. That mozilla bug is adding a new property to channel's loadinfo object (nsILoadInfo) that protocol handlers has to set on channels when originalURI on the result channel is set to a different URI than the channel has been created for.
Existence of the new property on nsILoadInfo depends on landing [1].
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1319111