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.
Re:Firefox Developer Edition
by vitaminjuk on Tuesday November 11, 2014 @06:09AM
(#8589282)
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-...