It is not advised to downgrade a profile and especially you shouldn't copy files from a Nightly profile to a release or Beta/DE/ESR profile as in Nightly a lot of features are enabled to make it possible to test whether it works properly or bugs need to be filed. Why would you want to do this ? You should always use a separate profile for each Firefox version. *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data