If you are referring to your account on this website, then you can use the password reset tool.

If you are referring to your Firefox account for sync and other services, you can use this password reset tool.

For both of the above, you will have to supply the email address that you used to create the account and you must have access to it in order to verify that you are the account holder. If you are unable to provide your email address on the password reset tool, then there is nothing else that we can do because Mozilla has no way of knowing if you are actually the intended account holder.

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.