Firefox has a feature to remember data you enter into forms, but it doesn't always match up. If you want to be able to put any data of your choice into any field of your choice, you will need an add-on. Over the years, independent developers have created numerous add-ons for Firefox to fill fields, both regular fields and password fields. Some of them are still going strong today, while others were broken by changes in Firefox and never fixed. As you consider different add-ons, please make sure to check the reviews to make sure they don't have recent problems or privacy concerns. Sample search: https://addons.mozilla.org/firefox/search/?q=fill+forms