An Important Change in Firefox: Dedicated Profiles
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Before, all Firefox installations were sharing a single profile by default. Now, Firefox provides dedicated profiles for each Firefox installation (this includes installations of Firefox Nightly, Firefox Beta, or Firefox ESR). This will make Firefox more stable when switching between Firefox installations on the same computer.
How Dedicated Profiles Work
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.
The installation you choose as your primary Firefox installation will keep your previously shared profile data like bookmarks, history and passwords.
You have not lost any personal data or customizations. Any previous profile data is safe and attached to the first Firefox installation that was opened after this change.
All of your Firefox installations will now have separate profiles.
What You Need to Do
If you do nothing, each installation will have a separate profile. This means your profile data will be different on each installation of Firefox.
You can change which installation uses which profile using the profile manager. Directions on how to use the profile manager are available on this support page.
While profiles are now connected to each installation of Firefox, you can use your Firefox Account to keep them in sync. If you would like all of your profile data to be the same on all installations of Firefox, we recommend you sync all of your profiles with a Firefox Account.