Quote:Mozilla plans to make changes to the profile handling of the Firefox browser in Firefox 65.  Firefox will switch to a dedicated profiles per install model in version 65 that assigns different profiles to each installation of the browser.

The system works similarly to how Firefox's Developer Edition handles this already; you can run Firefox's Developer Edition next to any other installed copy of Firefox because it uses a dedicated profile.

Tip: load about:profiles in Firefox for a detailed view of all existing profiles.

Firefox stores all user data in user profiles and the main user profile is shared across different channel versions of the web browser right now with the exception of the Developer Edition which uses a dedicated profile by default.

Using the same profile with different versions of Firefox may cause issues: from crashes on startup to broken functionality because of version changes. Two recent examples were the releases of Firefox 55 and Firefox 63 which users could not downgrade to earlier versions.

Full reading: https://www.ghacks.net/2018/10/27/firefox-65-dedicated-profiles-per-install/