Exciting stuff! If you're already on Aurora, when you auto-update to developer edition, you'll switch over to the new dev profile and your bookmarks and settings will be gone. You can get at those by opening the profile manager and switching back to the default profile, or by using a stable version of Firefox.
In my case this was unnecessary and the update to Developer Edition preserved everything. Might have something to do with the fact that I already make use of multiple profiles.
EDIT: Wait! The default profile was used when clicking "Restart" from within the updater, but when closing and reopening the browser manually it did indeed start up with a new profile.
You don't need to manually fiddle with profiles, just untick "Allow Firefox Developer Edition and Firefox to run at the same time" in settings to go back to your old profile rather than the segregated one
https://support.mozilla.org/en-US/kb/profile-manager-create-...