Bug 1565200 is going to provide the necessary implementation to debug Fission remote <iframe>, but preffed off behind devtools.fission.use-js-window-actor.

This bug is about turning this preference to true by default. Unless this preference is turned on, the FrameDescriptor.getTarget will fail creating target front for the fission iframes. So that TargetList won't notify about them. Leading to toolboxes not inspecting these documents.

This patch will impact all the toolboxes as we will switch from using the Message Manager API to the JS Window Actor API. But this has no visible impact. Except all the potential regressions of such low level change!
It won't allow content toolboxes to debug the fission remote iframe, unless bug 1578747 preference (devtools.contenttoolbox.fission) is set to true.
But it will immediately allow the Omniscient browser toolbox to debug fission remote iframes.

Tentatively moving all bugs whose summaries mention "Fission" (or other Fission-related keywords) but are not assigned to a Fission Milestone to the "?" triage milestone.

This will generate a lot of bugmail, so you can filter your bugmail for the following UUID and delete them en masse:

0ee3c76a-bc79-4eb2-8d12-05dc0b68e732

Blocks enabling Fission in Nightly (M6).

Our strategy changed a bit regarding JS Window Actors, and remote frame inspection in the MBT (Multiprocess Browser Toolbox :) ) is now handled in 1603209.

Should we close this one as duplicate?

Yes!