If you changed your password and need to update your saved date in Firefox, you can do so by clicking on the below article for step by step instructions. 

Please note: Computer Support does not recommend saving passwords in browsers for any sites. If it is necessary for you to use this feature, be sure your device is always locked, or secured when not in your immediate control. 


https://support.mozilla.org/en-US/kb/password-manager-remember-delete-change-and-import