Currently, whenever a messagemanager for a tab is created, a
tab-content-frameloader-created notification is fired from a framescript loaded by an extension. As we don't load framescripts for out-of-process subframes, this means that the observer notification to create
ExtensionGlobal objects (https://searchfox.org/mozilla-central/rev/3aef835f6cb12e607154d56d68726767172571e4/toolkit/components/extensions/ExtensionProcessScript.jsm#146-150) will never fire, and any functionality tied to these objects will be broken in extension code for out-of-process iframes.
It appears this functionality is largely used to implement
getFrameData which is used to fetch information about
viewType for a particular frame. These getters will likely behave incorrectly in cross-origin subframes.