If you are talking about your Password for a Firefox Account - you are the only one who ever knows your Password.

Go to the login page and use the 'Forgot Password' link.

However, if you are talking about sync;

Resetting the sync password, or changing its e-mail, will remove all data from the servers.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address