Assuming that your are talking about a Firefox Account password, see this KB article: https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

You can reset your password, but be aware that if you are using Sync your Firefox data which is saved on the Sync server your data will be cleared.

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.