The RDM UI is currently built in a way that isn't very Fission-friendly.
It simulates a browser viewport by using an <iframe mozbrowser> element that is loaded in a normal browser tab. Various messages that the browser uses are relayed (using the message manager) between the browser and the simulated viewport.
As the browser front-end's messages are getting refactored piece by piece to make use of new actors, this makes it harder and harder for RDM to work.
The plan is to remove the need for the message-manager-heavy architecture.
See the description of impacts and meeting transcripts here: https://docs.google.com/document/d/1lwZLQj-pwymD0-gAMt7b3UF6jVLE-MGUMvpRc2Kxj-c/edit#heading=h.8147ii4syt9c