What is changing?
Firefox saves information such as bookmarks, passwords and user preferences in a set of files called your profile. This profile is stored in a separate location from the Firefox program files. Learn more about profiles.
Previously, all Firefox installations shared a single profile by default. On January 28, Firefox will begin creating dedicated profiles for each installation, starting with Firefox 67. On that date, Firefox will begin using a dedicated profile for Firefox Nightly, and this Nightly installation will no longer share a profile with other Firefox installations on your computer. Other versions of Firefox (Beta, ESR and Firefox) will have a dedicated profile once they reach release 67.
This will make Firefox more stable when switching between installations on the same computer and will also allow you to run different installations of Firefox at the same time.
You already use a separate profile for Nightly?
Users who have already manually created separate profiles for different installations will not notice the change (this has been the advised procedure on Nightly for a while).
What should I do?
If you do nothing and are currently running different installations of Firefox that share the same profile, your Nightly installation will start running a dedicated profile on January 28th. If Nightly is the first installation of Firefox you open on or after January 28, it will use your existing profile. If Nightly is not the first installation you open, Nightly will get a new profile. Your existing profile will remain associated with the first installation you open on or after January 28.
If you would like all of your profile data to be the same on all installations of Firefox, you can use a Firefox Account to keep them in sync.
A Firefox Account is the easiest way to make your profiles consistent on all of your versions of Firefox. You also get additional benefits like sending tabs and secure password storage. Get started with a Firefox Account.