But you did ok by creating another Firefox account and avoided some cleanup work of outdated logon credentials.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and then you would have to choose between two sets of credentials until you deleted the old, probably nonworking outdated one from Firefox's saved passwords.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.
