What are profiles?

All of a Firefox user’ settings are stored in what we call the user profile. More details on what profiles are can be found here.

What changes did we make?

Switching between newer and older installs of Firefox using the same profile on the same computer potentially causes profile corruption and data loss because changes in the profile structure are typically not backward compatible.

Starting with Firefox 62, users switching between multiple builds installed in different locations on disk with the same profile will see the following warnings:

• When switching from an older build to a newer build, you won't be able to switch back without changing profiles.
• When switching from a newer build to an older build, you'll be advised to run the most recent build or use a different profile to run the older build.

What are the implications for those who don’t share profiles between different Firefox installs?

None. Users with a single Firefox installation or with several installations having each a separate profile won’t see the warning.

Can I use Sync to still access my bookmarks and browsing history on different Firefox installs?

If you are a signed-in Sync user on the newer version of Firefox, when starting up the older version of Firefox, you can open the Profile Manager and choose a different profile. Afterwards, sign in to your Firefox Account and perform a sync to restore your bookmarks and browsing history.