Would you please take a look at this :

https://support.mozilla.org/en-US/kb/reset-your-master-password-if-you-forgot-it

But make sure you read the warning ......

https://support.mozilla.org/kb/Forgot+my+master+password Resetting your master password will remove all of your saved usernames and passwords. This is a security feature.

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.