The devil is in the details though: https://hg-edge.mozilla.org/integration/autoland/diff/8a6d6c...
Looking briefly at this,
* there doesn't appear to be any migration from old directory to new directory. Does the code just use ~/.mozilla if it still exists, ~/.config/mozilla otherwise.. or does it _require_ MOZ_LEGACY_HOME=1 to be set to keep using your existing config, and just lose all config if you don't set that?
* there doesn't appear to be a proper split between ~/.cache (always-removable cached data) ~/.config (configuration) and ~/.local/share (application data that is not user-editable configuration and is not just cached data either), they just moved the entire set of profile stuff to ~/.config
Is that about right, or do I need to read the code more carefully?