Later this week, we’ll be enabling a new version of the Responsive Design Mode tool that no longer relies on <iframe mozbrowser> as part of the initiative to make DevTools Fission-compatible. For now, the pref will only be enabled in Firefox Nightly as we work with QA to ensure the stability of the tool.
This new version of RDM will be embedded inside the browser UI (rather than live in a tab like today) and website content will be rendered inside the <browser remote> element just like normal. Users should not expect any significant difference between this version and the current one.
Tracking bug for RDM Fission: https://bugzilla.mozilla.org/show_bug.cgi?id=1574885. We ask anyone finding anything wrong with the new tool once the pref is on to report bugs blocking this meta.