The definition of "Aurora/Nightly" used here includes builds shipped on the Aurora/Nightly update channels by Mozilla releng, but *excludes* builds built locally by developers.
- if I download and start Aurora/Nightly on a system where only one Firefox profile exists, Aurora/Nightly will create a new profile ("default channel profile") and use it automatically in subsequent browsing sessions. A system-specific pref (stored in profiles.ini?) will be set to put this user in "new profile mode".
- when I start up Aurora/Nightly in "new profile mode", it will always use the default channel profile
- there will be a checkbox in the Aurora/Nightly preferences pane (or profile manager?) that toggles "old profile mode"/"new profile mode"
- if an existing Aurora/Nightly user (or existing user with multiple profiles) switches to "new profile mode" (via the checkbox), they will be prompted to restart Aurora/Nightly, and upon restart the build will use the default channel profile
- if a "new profile mode" user turns off "new profile mode" in Nightly/Aurora, upon restart they will get "old profile mode" behavior
- the -P <profile> or -profile command line arguments will continue to work regardless of whether the user is in "new profile mode" or "old profile mode"