See:

• https://support.mozilla.org/en-US/kb/use-primary-password-protect-stored-logins
• https://support.mozilla.org/en-US/kb/reset-your-primary-password-if-youve-forgotten-it

Are you using Sync as this is a possible reason to ask for the Primary Password at startup ?

Using Sync is a possible reason to ask for the Primary Password at startup. This is because the credentials to login to the Sync account are stored in the Password Manager and you need to enter the Primary Password to unlock the passwords.

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