When you Sign In to Firefox, you are signing in with YOUR Email Address with YOUR password. No one else should have this account.
All your settings, bookmarks, passwords etc will remain the same.
All that will happen, is when you login to Firefox from another device with YOUR Email address and YOUR password, all of those settings will appear on that device. They will not disappear from your origina device.
If Sync is turned on, any New Settings etc that you change or Save after that will also appear on any other device with Firefox that YOU sign into.

Of course, you can always Backup your Firefox Profile in case things go awry. Back up and transfer your Firefox data to a new device | Firefox Help

 

Thanks, nothing bad happened. I created a new account to sync and I didn't lose any data.

 

But you did ok by creating another Firefox account and avoided some cleanup work of outdated logon credentials.

If you are not signed in but have Firefox remembering bookmarks and passwords AND THEN you sign in to an already existing account you used before, it will ADD the old past-remembered bookmarks and passwords to your current and already saved bookmarks and passwords. This would create duplicate logons which would be apparent in sign-on screens and then you would have to choose between two sets of credentials until you deleted the old, probably nonworking outdated one from Firefox's saved passwords.

You did it smart to avoid the scenario I described. Another way to do this but keep your same Firefox sync logon is to use a different computer, install Firefox, sign in with your desired account. Then delete the bookmarks in Bookmark Manager and also go into passwords to delete all the entries. Be sure Sync stayed on during all this. Then sign out of Firefox and choose the option to delete your data to remove history and cookies. Now go to your usual computer with the current saved bookmarks and passwords and sign in to Sync.